Re: [Intel-gfx] [PATCH] drm/i915/gt: Add separate MOCS table for Gen12 devices other than TGL/RKL
On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote:
On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote: > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote: > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote: > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote: > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC. > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB, > > > > So adding a new MOCS table for other gen 12 devices eg. ADL. > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values") > > > > Cc: Matt Roper matthew.d.roper@intel.com > > > > Signed-off-by: Ayaz A Siddiqui ayaz.siddiqui@intel.com > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup > > > for MOCS[1]. The new table is the same as the TGL table, just with > > > I915_MOCS_PTE (1) removed. > > > > And just how are people planning on handling display cacheability > > control without a PTE MOCS entry? Is Mesa/etc. already making all > > external bos uncached on these platforms just in case we might > > scan out said bo? > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12 > platforms (despite the old #define, it's not actually related to PTE, > display, etc. anymore).
So can someone finally explain to me how we're supposed to cache anything that might become a scanout buffer later (eg. window system buffers)? Or are we just making everything like that UC now, and is everyone happy with that? Is userspace actually following that?
Table entry #1 has never had anything to do with scanout on gen12+. I would assume that UMDs are either using the display entry in the MOCS table (which is 61 on gen12+) or some other UC entry.
If 61 is meant to to be the new PTE entry wy hasn't it been defines as such in the code? And I know for a fact that userspace (Mesa) is not
There is no "PTE" entry anymore. But 61 is already documented as "displayable" in both the spec and the code:
/* HW Special Case (Displayable) */ MOCS_ENTRY(61,Why is it called a "HW special case"? I don't think there's any hw magic in there?
And why aren't we setting it to PTE to get some cacheability for window back buffers and such?
Who is "we" here?
We who care about the performance of the system.
The MOCS table is a pre-defined set of per-platform magic numbers. The software teams don't get to decide what the values are, we just program the hardware with the per-platform numbers that have been agreed upon as part of a platform-wide stack (everything from low-level firmware to high level userspace should be working from the same table, defined in the bspec).
The magic numbers must be based on something. If that something is purely Windows behaviour/performance then we might be shooting ourselves in the foot here.
Once we know what the per-platform magic numbers are, we're supposed to pick the table entry that matches the behavior we're trying to accomplish. If you want some specific level of cacheability, then you select a table row that gives you that. Maybe 61 isn't the best setting, I don't know; userspace can pick whichever defined setting is actually best, using the data from the table. But table row #1 is already well-documented as reserved/dontuse across the full stack; the fact that row #1 had values similar to PTE on Icelake hardware doesn't carry forward to any post-gen11 platform.
The only way you can get LLC cacheability for an external BO (window back buffers and such) is by using a MOCS entry that directs the hardware to consult the PTEs. Otherwise the client doing the rendering would have to know ahead of time whether the buffer is going to be directly scanned out by the compositor or not, for which there is no protocol in X or wayland.
Historically I believe LLC cacheability has been on average a win. Some workloads can do better with UC though. So if we are giving up on LLC cacheability we should have some numbers to back up that decision so that we're not dropping tons of performance on the floor.
using entry 61. I think there is a massive communication gap here where everyone just seems to assume the other side is doing something.
Could someone actually come up with a clear abi definition for this and get all the stakeholders to sign off on it?
The agreement between the i915 team, various userspace teams, Windows driver team, hardware architects, software architects, and bspec writers was just completed; that's what triggered the kernel updates here (and I'm guessing is triggering similar work on the UMD side). It's also why we held off on removing the force_probe flag on ADL until now since we couldn't consider enablement of the platform complete until the agreement and definitions here was finalized.
Can we get that agreement visible on the mailing list? Since MOCS is abi I don't see why we shouldn't follow the normal abi rules for these, ie. post to dri-devel, get acks from relevant people, links to agreed userspace changes, etc.
The ABI design here was designed and agreed upon years ago, during early gen11 development. The ABI design is that the kernel driver will faithfully initialize the hardware with the pre-determined set of magic numbers documented by the hardware team. Since these are well-documented and unchanging numbers per-platform, there's no ambiguity for userspace, firmware, etc. about what a specific mocs index means, and no need to provide additional ABI for userspace to query what the kernel used in each row or anything like that. The specific magic numbers are also ABI in the sense that we can't change the set of defined values once they're set for a platform (and it's been a long road to get the hardware and other OS software teams to understand and agree to this requirement), but we don't get to define or overrule what the initial values and order of those magic numbers are.
And this apporach has clearly not worked considering userspace and kernel have not agreed on what the abi is. We need to do better.
What is a bit vague in the formal documentation is what should be done about the reserved/dontuse table entries. In theory it wouldn't matter since they'd never be used anyway, but in reality userspace can still use them by accident, such as by forgetting to update their MOCS selection logic from past platforms (e.g., still trying to use row #1 on platforms where it isn't defined). Given that it's legal for entries to be added to MOCS tables, but never removed/modified, it follows that we should always initialize the undefined entries to fully cached; if a MOCS table update happens in the future and new rows show up, they can only become more coherent, and any userspace software that was incorrectly trying to use them previously will remain functionally correct.
What you're proposing would be a change to existing ABI --- instead of following the agreed upon contract, i915 would start defining its own set of magic numbers that potentially contradict the documentation that every other team is depending on. We already made this mistake on TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever becomes a formally defined setting in the future, the rest of the software stack will need to explicitly work around i915's bug since we can't fix it now without breaking ABI.
If you really want to redefine how the MOCS ABI works and have i915 no longer follow the current contract, I think you need to do the following:
I want the abi to be actually defined properly, and some assurance that all the stakeholders implement it correctly. Following the proper abi rules for kernel development would guarantee that.
I would also like if the abi can give us the best performance rather than potentially crippling it. Ie. I would expect to have a PTE MOCS setting for potential scanout buffers, or some proof that pure UC will in fact be a better choice.
- Re-add force_probe requirement to ADL and add appropriate Fixes: so that the platform isn't enabled yet.
- Document clearly how you want i915 to select the MOCS settings it uses for each table row if we're no longer going to follow the documented values in the bspec.
- Provide a way for userspace to determine how i915 has defined the MOCS settings (since they can no longer rely on us following the previously agreed upon contract).
- Get acks from all the userspace teams on the new direction you're proposing.
- Get an ack from the GuC team to make sure that programming MOCS values differently than documented in the bspec won't have any kind of impact on their operation.
Alternatively, you could lobby for a new table row #1 to be added to the formal MOCS table for gen12. It's legal for new MOCS entries to show up in the future, so if that happens, we can follow up with the corresponding change in i915; since we'll be moving to a more coherent value (from today's fully cached entry), we'll be becoming more permissive from a correctness point of view. But we absolutely should not try to add entries unilaterally in i915 that haven't been formally agreed upon because they may clash with a different definition of the row that shows up in the future through formal channels.
On Thu, Sep 09, 2021 at 08:42:15PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote: > On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote: > > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote: > > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote: > > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote: > > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC. > > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB, > > > > > So adding a new MOCS table for other gen 12 devices eg. ADL. > > > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values") > > > > > Cc: Matt Roper matthew.d.roper@intel.com > > > > > Signed-off-by: Ayaz A Siddiqui ayaz.siddiqui@intel.com > > > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for > > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup > > > > for MOCS[1]. The new table is the same as the TGL table, just with > > > > I915_MOCS_PTE (1) removed. > > > > > > And just how are people planning on handling display cacheability > > > control without a PTE MOCS entry? Is Mesa/etc. already making all > > > external bos uncached on these platforms just in case we might > > > scan out said bo? > > > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12 > > platforms (despite the old #define, it's not actually related to PTE, > > display, etc. anymore). > > So can someone finally explain to me how we're supposed to cache > anything that might become a scanout buffer later (eg. window system > buffers)? Or are we just making everything like that UC now, and is > everyone happy with that? Is userspace actually following that?
Table entry #1 has never had anything to do with scanout on gen12+. I would assume that UMDs are either using the display entry in the MOCS table (which is 61 on gen12+) or some other UC entry.
If 61 is meant to to be the new PTE entry wy hasn't it been defines as such in the code? And I know for a fact that userspace (Mesa) is not
There is no "PTE" entry anymore. But 61 is already documented as "displayable" in both the spec and the code:
/* HW Special Case (Displayable) */ MOCS_ENTRY(61,Why is it called a "HW special case"? I don't think there's any hw magic in there?
And why aren't we setting it to PTE to get some cacheability for window back buffers and such?
Who is "we" here?
We who care about the performance of the system.
The MOCS table is a pre-defined set of per-platform magic numbers. The software teams don't get to decide what the values are, we just program the hardware with the per-platform numbers that have been agreed upon as part of a platform-wide stack (everything from low-level firmware to high level userspace should be working from the same table, defined in the bspec).
The magic numbers must be based on something. If that something is purely Windows behaviour/performance then we might be shooting ourselves in the foot here.
That's not how MOCS works. The MOCS tables define every meaningful combination of settings somewhere in the table. The *types* of settings that can be expressed change from platform to platform (e.g., "PAGETABLE" setting simply doesn't exist anymore hardware-wise) so the tables themselves differ between platforms and you may need to use different indices to get the same behavior between platforms. But if you're actually paying attention to the tables and choosing the right entries, you're not going to leave any performance on the table.
Once we know what the per-platform magic numbers are, we're supposed to pick the table entry that matches the behavior we're trying to accomplish. If you want some specific level of cacheability, then you select a table row that gives you that. Maybe 61 isn't the best setting, I don't know; userspace can pick whichever defined setting is actually best, using the data from the table. But table row #1 is already well-documented as reserved/dontuse across the full stack; the fact that row #1 had values similar to PTE on Icelake hardware doesn't carry forward to any post-gen11 platform.
The only way you can get LLC cacheability for an external BO (window back buffers and such) is by using a MOCS entry that directs the hardware to consult the PTEs. Otherwise the client doing the rendering would have to know ahead of time whether the buffer is going to be directly scanned out by the compositor or not, for which there is no protocol in X or wayland.
It sounds like you're complaining about gen12 hardware design rather than anything actually software or ABI related. 'Consult the PTEs' hasn't been something the hardware has supported since gen11. We still have macros with PTE or PAGETABLE in their names, but those aren't accurate descriptions of what they do on current platforms. We note this in the code:
/* * Note: LE_0_PAGETABLE works only up to Gen11; for newer gens it means * the same as LE_UC */
Even on gen11 the PAT entry was documented as being deprecated, so we knew it was going away. I don't know why the hardware design changed, but it did.
Historically I believe LLC cacheability has been on average a win. Some workloads can do better with UC though. So if we are giving up on LLC cacheability we should have some numbers to back up that decision so that we're not dropping tons of performance on the floor.
It's not our decision to make. The hardware design has already made this decision for us. No matter what we program into the MOCS tables; we're simply not getting back the behavior you're asking for and blindly using the "LE_0_PAGETABLE" flags or whatever is just giving you uncached.
using entry 61. I think there is a massive communication gap here where everyone just seems to assume the other side is doing something.
Could someone actually come up with a clear abi definition for this and get all the stakeholders to sign off on it?
The agreement between the i915 team, various userspace teams, Windows driver team, hardware architects, software architects, and bspec writers was just completed; that's what triggered the kernel updates here (and I'm guessing is triggering similar work on the UMD side). It's also why we held off on removing the force_probe flag on ADL until now since we couldn't consider enablement of the platform complete until the agreement and definitions here was finalized.
Can we get that agreement visible on the mailing list? Since MOCS is abi I don't see why we shouldn't follow the normal abi rules for these, ie. post to dri-devel, get acks from relevant people, links to agreed userspace changes, etc.
The ABI design here was designed and agreed upon years ago, during early gen11 development. The ABI design is that the kernel driver will faithfully initialize the hardware with the pre-determined set of magic numbers documented by the hardware team. Since these are well-documented and unchanging numbers per-platform, there's no ambiguity for userspace, firmware, etc. about what a specific mocs index means, and no need to provide additional ABI for userspace to query what the kernel used in each row or anything like that. The specific magic numbers are also ABI in the sense that we can't change the set of defined values once they're set for a platform (and it's been a long road to get the hardware and other OS software teams to understand and agree to this requirement), but we don't get to define or overrule what the initial values and order of those magic numbers are.
And this apporach has clearly not worked considering userspace and kernel have not agreed on what the abi is. We need to do better.
Userspace and kernel *do* agree on what the ABI is. There may be a delay in userspace enabling the MOCS properly on a new platform. But the changes here (which make everything fully cached by default) will help catch those cases where a bit of enablement was forgotten on the UMD side. They won't "accidentally work" on initial bringup on the new platform, and the lack of proper MOCS programming will be quickly identified.
What is a bit vague in the formal documentation is what should be done about the reserved/dontuse table entries. In theory it wouldn't matter since they'd never be used anyway, but in reality userspace can still use them by accident, such as by forgetting to update their MOCS selection logic from past platforms (e.g., still trying to use row #1 on platforms where it isn't defined). Given that it's legal for entries to be added to MOCS tables, but never removed/modified, it follows that we should always initialize the undefined entries to fully cached; if a MOCS table update happens in the future and new rows show up, they can only become more coherent, and any userspace software that was incorrectly trying to use them previously will remain functionally correct.
What you're proposing would be a change to existing ABI --- instead of following the agreed upon contract, i915 would start defining its own set of magic numbers that potentially contradict the documentation that every other team is depending on. We already made this mistake on TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever becomes a formally defined setting in the future, the rest of the software stack will need to explicitly work around i915's bug since we can't fix it now without breaking ABI.
If you really want to redefine how the MOCS ABI works and have i915 no longer follow the current contract, I think you need to do the following:
I want the abi to be actually defined properly, and some assurance that all the stakeholders implement it correctly. Following the proper abi rules for kernel development would guarantee that.
This is what we have today. As I said, if you want to propose a change to the ABI, then you need to take the proper steps to do so. But as far as I can tell, most of your concern here boils down to not realizing that the hardware capabilities you want to use were taken away from us in gen12.
I would also like if the abi can give us the best performance rather than potentially crippling it. Ie. I would expect to have a PTE MOCS setting for potential scanout buffers, or some proof that pure UC will in fact be a better choice.
Again, this isn't a choice. This is a fact of how the hardware works on gen12. Maybe you'll regain PTE flags on a future platform, but that's simply not something software can ask for on today's platforms.
Matt
- Re-add force_probe requirement to ADL and add appropriate Fixes: so that the platform isn't enabled yet.
- Document clearly how you want i915 to select the MOCS settings it uses for each table row if we're no longer going to follow the documented values in the bspec.
- Provide a way for userspace to determine how i915 has defined the MOCS settings (since they can no longer rely on us following the previously agreed upon contract).
- Get acks from all the userspace teams on the new direction you're proposing.
- Get an ack from the GuC team to make sure that programming MOCS values differently than documented in the bspec won't have any kind of impact on their operation.
Alternatively, you could lobby for a new table row #1 to be added to the formal MOCS table for gen12. It's legal for new MOCS entries to show up in the future, so if that happens, we can follow up with the corresponding change in i915; since we'll be moving to a more coherent value (from today's fully cached entry), we'll be becoming more permissive from a correctness point of view. But we absolutely should not try to add entries unilaterally in i915 that haven't been formally agreed upon because they may clash with a different definition of the row that shows up in the future through formal channels.
-- Ville Syrjälä Intel
On Thu, Sep 09, 2021 at 11:14:15AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 08:42:15PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote: > On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote: > > On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote: > > > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote: > > > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote: > > > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote: > > > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC. > > > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB, > > > > > > So adding a new MOCS table for other gen 12 devices eg. ADL. > > > > > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values") > > > > > > Cc: Matt Roper matthew.d.roper@intel.com > > > > > > Signed-off-by: Ayaz A Siddiqui ayaz.siddiqui@intel.com > > > > > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for > > > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup > > > > > for MOCS[1]. The new table is the same as the TGL table, just with > > > > > I915_MOCS_PTE (1) removed. > > > > > > > > And just how are people planning on handling display cacheability > > > > control without a PTE MOCS entry? Is Mesa/etc. already making all > > > > external bos uncached on these platforms just in case we might > > > > scan out said bo? > > > > > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12 > > > platforms (despite the old #define, it's not actually related to PTE, > > > display, etc. anymore). > > > > So can someone finally explain to me how we're supposed to cache > > anything that might become a scanout buffer later (eg. window system > > buffers)? Or are we just making everything like that UC now, and is > > everyone happy with that? Is userspace actually following that? > > Table entry #1 has never had anything to do with scanout on gen12+. I > would assume that UMDs are either using the display entry in the MOCS > table (which is 61 on gen12+) or some other UC entry.
If 61 is meant to to be the new PTE entry wy hasn't it been defines as such in the code? And I know for a fact that userspace (Mesa) is not
There is no "PTE" entry anymore. But 61 is already documented as "displayable" in both the spec and the code:
/* HW Special Case (Displayable) */ MOCS_ENTRY(61,Why is it called a "HW special case"? I don't think there's any hw magic in there?
And why aren't we setting it to PTE to get some cacheability for window back buffers and such?
Who is "we" here?
We who care about the performance of the system.
The MOCS table is a pre-defined set of per-platform magic numbers. The software teams don't get to decide what the values are, we just program the hardware with the per-platform numbers that have been agreed upon as part of a platform-wide stack (everything from low-level firmware to high level userspace should be working from the same table, defined in the bspec).
The magic numbers must be based on something. If that something is purely Windows behaviour/performance then we might be shooting ourselves in the foot here.
That's not how MOCS works. The MOCS tables define every meaningful combination of settings somewhere in the table. The *types* of settings that can be expressed change from platform to platform (e.g., "PAGETABLE" setting simply doesn't exist anymore hardware-wise) so the tables themselves differ between platforms and you may need to use different indices to get the same behavior between platforms. But if you're actually paying attention to the tables and choosing the right entries, you're not going to leave any performance on the table.
Once we know what the per-platform magic numbers are, we're supposed to pick the table entry that matches the behavior we're trying to accomplish. If you want some specific level of cacheability, then you select a table row that gives you that. Maybe 61 isn't the best setting, I don't know; userspace can pick whichever defined setting is actually best, using the data from the table. But table row #1 is already well-documented as reserved/dontuse across the full stack; the fact that row #1 had values similar to PTE on Icelake hardware doesn't carry forward to any post-gen11 platform.
The only way you can get LLC cacheability for an external BO (window back buffers and such) is by using a MOCS entry that directs the hardware to consult the PTEs. Otherwise the client doing the rendering would have to know ahead of time whether the buffer is going to be directly scanned out by the compositor or not, for which there is no protocol in X or wayland.
It sounds like you're complaining about gen12 hardware design rather than anything actually software or ABI related. 'Consult the PTEs' hasn't been something the hardware has supported since gen11. We still have macros with PTE or PAGETABLE in their names, but those aren't accurate descriptions of what they do on current platforms.
I've heard this a few times, but so far I've not been able to find any evidence for it in bspec. I suppose to find out for sure I'm going to have to test it on actual hardware.
I did stumble on some future platform stuff that shuffles around the MOCS stuff once again, and looks like it adds in a specific PTE vs. MOCS bit. So if it was removed, it's almost immediately coming back.
We note this in the code:
/* * Note: LE_0_PAGETABLE works only up to Gen11; for newer gens it means * the same as LE_UC */
Even on gen11 the PAT entry was documented as being deprecated, so we knew it was going away. I don't know why the hardware design changed, but it did.
Yeah, I've seen it, but could never figure out on what it was based.
Historically I believe LLC cacheability has been on average a win. Some workloads can do better with UC though. So if we are giving up on LLC cacheability we should have some numbers to back up that decision so that we're not dropping tons of performance on the floor.
It's not our decision to make. The hardware design has already made this decision for us. No matter what we program into the MOCS tables; we're simply not getting back the behavior you're asking for and blindly using the "LE_0_PAGETABLE" flags or whatever is just giving you uncached.
As said it looks to be coming back soon, if it ever went away.
using entry 61. I think there is a massive communication gap here where everyone just seems to assume the other side is doing something.
Could someone actually come up with a clear abi definition for this and get all the stakeholders to sign off on it?
The agreement between the i915 team, various userspace teams, Windows driver team, hardware architects, software architects, and bspec writers was just completed; that's what triggered the kernel updates here (and I'm guessing is triggering similar work on the UMD side). It's also why we held off on removing the force_probe flag on ADL until now since we couldn't consider enablement of the platform complete until the agreement and definitions here was finalized.
Can we get that agreement visible on the mailing list? Since MOCS is abi I don't see why we shouldn't follow the normal abi rules for these, ie. post to dri-devel, get acks from relevant people, links to agreed userspace changes, etc.
The ABI design here was designed and agreed upon years ago, during early gen11 development. The ABI design is that the kernel driver will faithfully initialize the hardware with the pre-determined set of magic numbers documented by the hardware team. Since these are well-documented and unchanging numbers per-platform, there's no ambiguity for userspace, firmware, etc. about what a specific mocs index means, and no need to provide additional ABI for userspace to query what the kernel used in each row or anything like that. The specific magic numbers are also ABI in the sense that we can't change the set of defined values once they're set for a platform (and it's been a long road to get the hardware and other OS software teams to understand and agree to this requirement), but we don't get to define or overrule what the initial values and order of those magic numbers are.
And this apporach has clearly not worked considering userspace and kernel have not agreed on what the abi is. We need to do better.
Userspace and kernel *do* agree on what the ABI is. There may be a delay in userspace enabling the MOCS properly on a new platform.
The fact that Mesa never started to use the MOCS entry supposedly meant for display stuff is evidence to the contrary.
But the changes here (which make everything fully cached by default) will help catch those cases where a bit of enablement was forgotten on the UMD side. They won't "accidentally work" on initial bringup on the new platform, and the lack of proper MOCS programming will be quickly identified.
I'm looking at the history and not really convinced.
What is a bit vague in the formal documentation is what should be done about the reserved/dontuse table entries. In theory it wouldn't matter since they'd never be used anyway, but in reality userspace can still use them by accident, such as by forgetting to update their MOCS selection logic from past platforms (e.g., still trying to use row #1 on platforms where it isn't defined). Given that it's legal for entries to be added to MOCS tables, but never removed/modified, it follows that we should always initialize the undefined entries to fully cached; if a MOCS table update happens in the future and new rows show up, they can only become more coherent, and any userspace software that was incorrectly trying to use them previously will remain functionally correct.
What you're proposing would be a change to existing ABI --- instead of following the agreed upon contract, i915 would start defining its own set of magic numbers that potentially contradict the documentation that every other team is depending on. We already made this mistake on TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever becomes a formally defined setting in the future, the rest of the software stack will need to explicitly work around i915's bug since we can't fix it now without breaking ABI.
If you really want to redefine how the MOCS ABI works and have i915 no longer follow the current contract, I think you need to do the following:
I want the abi to be actually defined properly, and some assurance that all the stakeholders implement it correctly. Following the proper abi rules for kernel development would guarantee that.
This is what we have today. As I said, if you want to propose a change to the ABI, then you need to take the proper steps to do so. But as far as I can tell, most of your concern here boils down to not realizing that the hardware capabilities you want to use were taken away from us in gen12.
The fact that it's not actually properly documented anywhere certainly doesn't help in figuring out what is going on.
I would also like if the abi can give us the best performance rather than potentially crippling it. Ie. I would expect to have a PTE MOCS setting for potential scanout buffers, or some proof that pure UC will in fact be a better choice.
Again, this isn't a choice. This is a fact of how the hardware works on gen12. Maybe you'll regain PTE flags on a future platform, but that's simply not something software can ask for on today's platforms.
There should be clear guidelines what MOCS index userspace is supposed to use in each case. That more or less used to be the case with the handful of MOCS entries we had defined (always uncached vs. always cached vs. potential scanout buffers which could be either depending on the situation). But that nice simple approach wasn't good enough for whatever reason.
On Thu, Sep 09, 2021 at 10:59:36PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 11:14:15AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 08:42:15PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote: > On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote: > > On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote: > > > On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote: > > > > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote: > > > > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote: > > > > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote: > > > > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC. > > > > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB, > > > > > > > So adding a new MOCS table for other gen 12 devices eg. ADL. > > > > > > > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values") > > > > > > > Cc: Matt Roper matthew.d.roper@intel.com > > > > > > > Signed-off-by: Ayaz A Siddiqui ayaz.siddiqui@intel.com > > > > > > > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for > > > > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup > > > > > > for MOCS[1]. The new table is the same as the TGL table, just with > > > > > > I915_MOCS_PTE (1) removed. > > > > > > > > > > And just how are people planning on handling display cacheability > > > > > control without a PTE MOCS entry? Is Mesa/etc. already making all > > > > > external bos uncached on these platforms just in case we might > > > > > scan out said bo? > > > > > > > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12 > > > > platforms (despite the old #define, it's not actually related to PTE, > > > > display, etc. anymore). > > > > > > So can someone finally explain to me how we're supposed to cache > > > anything that might become a scanout buffer later (eg. window system > > > buffers)? Or are we just making everything like that UC now, and is > > > everyone happy with that? Is userspace actually following that? > > > > Table entry #1 has never had anything to do with scanout on gen12+. I > > would assume that UMDs are either using the display entry in the MOCS > > table (which is 61 on gen12+) or some other UC entry. > > If 61 is meant to to be the new PTE entry wy hasn't it been defines as > such in the code? And I know for a fact that userspace (Mesa) is not
There is no "PTE" entry anymore. But 61 is already documented as "displayable" in both the spec and the code:
/* HW Special Case (Displayable) */ MOCS_ENTRY(61,Why is it called a "HW special case"? I don't think there's any hw magic in there?
And why aren't we setting it to PTE to get some cacheability for window back buffers and such?
Who is "we" here?
We who care about the performance of the system.
The MOCS table is a pre-defined set of per-platform magic numbers. The software teams don't get to decide what the values are, we just program the hardware with the per-platform numbers that have been agreed upon as part of a platform-wide stack (everything from low-level firmware to high level userspace should be working from the same table, defined in the bspec).
The magic numbers must be based on something. If that something is purely Windows behaviour/performance then we might be shooting ourselves in the foot here.
That's not how MOCS works. The MOCS tables define every meaningful combination of settings somewhere in the table. The *types* of settings that can be expressed change from platform to platform (e.g., "PAGETABLE" setting simply doesn't exist anymore hardware-wise) so the tables themselves differ between platforms and you may need to use different indices to get the same behavior between platforms. But if you're actually paying attention to the tables and choosing the right entries, you're not going to leave any performance on the table.
Once we know what the per-platform magic numbers are, we're supposed to pick the table entry that matches the behavior we're trying to accomplish. If you want some specific level of cacheability, then you select a table row that gives you that. Maybe 61 isn't the best setting, I don't know; userspace can pick whichever defined setting is actually best, using the data from the table. But table row #1 is already well-documented as reserved/dontuse across the full stack; the fact that row #1 had values similar to PTE on Icelake hardware doesn't carry forward to any post-gen11 platform.
The only way you can get LLC cacheability for an external BO (window back buffers and such) is by using a MOCS entry that directs the hardware to consult the PTEs. Otherwise the client doing the rendering would have to know ahead of time whether the buffer is going to be directly scanned out by the compositor or not, for which there is no protocol in X or wayland.
It sounds like you're complaining about gen12 hardware design rather than anything actually software or ABI related. 'Consult the PTEs' hasn't been something the hardware has supported since gen11. We still have macros with PTE or PAGETABLE in their names, but those aren't accurate descriptions of what they do on current platforms.
I've heard this a few times, but so far I've not been able to find any evidence for it in bspec. I suppose to find out for sure I'm going to have to test it on actual hardware.
For gen11, bspec page 560 describes the layout of the GFX_MOCS_x registers (which at the time were per-engine). Specifically:
Memory type information used in LLC/eDRAM.
00b: Use Cacheability Controls from page table / UC with Fence (if coherent cycle). 01b: Uncacheable (UC) – non-cacheable. 10b: Writethrough (WT). 11b: Writeback (WB).
So on that platform you can get the PTE behavior you're after by programming a value of 00 into the relevant bits (1:0).
For gen12+, bspec page 44053 describes the layout of GLOB_MOCS_LECC_x (which is now a global value, not per-engine):
Memory type information used in LLC/eDRAM.
00: Uncacheable (UC) 01: Uncacheable (WC) 10: Writethrough (WT) 11: Writeback (WB)
So the value of '00' as expressed in a MOCS table no longer means what it did on older platforms. There's also a "Description" field on this page that shows that the old meaning of 00 is explicitly removed by gen12 design.
I did stumble on some future platform stuff that shuffles around the MOCS stuff once again, and looks like it adds in a specific PTE vs. MOCS bit. So if it was removed, it's almost immediately coming back.
Yes, bspec page 65553 makes it look like there is a new bit to handle this eventually on a future platform. But we have multiple new platforms between now and then that we still need to support properly, starting with ADL, and that bit (and the rest of the various MOCS changes) aren't something we can use yet.
We note this in the code:
/* * Note: LE_0_PAGETABLE works only up to Gen11; for newer gens it means * the same as LE_UC */
Even on gen11 the PAT entry was documented as being deprecated, so we knew it was going away. I don't know why the hardware design changed, but it did.
Yeah, I've seen it, but could never figure out on what it was based.
Historically I believe LLC cacheability has been on average a win. Some workloads can do better with UC though. So if we are giving up on LLC cacheability we should have some numbers to back up that decision so that we're not dropping tons of performance on the floor.
It's not our decision to make. The hardware design has already made this decision for us. No matter what we program into the MOCS tables; we're simply not getting back the behavior you're asking for and blindly using the "LE_0_PAGETABLE" flags or whatever is just giving you uncached.
As said it looks to be coming back soon, if it ever went away.
> using entry 61. I think there is a massive communication gap here > where everyone just seems to assume the other side is doing something. > > Could someone actually come up with a clear abi definition for this > and get all the stakeholders to sign off on it?
The agreement between the i915 team, various userspace teams, Windows driver team, hardware architects, software architects, and bspec writers was just completed; that's what triggered the kernel updates here (and I'm guessing is triggering similar work on the UMD side). It's also why we held off on removing the force_probe flag on ADL until now since we couldn't consider enablement of the platform complete until the agreement and definitions here was finalized.
Can we get that agreement visible on the mailing list? Since MOCS is abi I don't see why we shouldn't follow the normal abi rules for these, ie. post to dri-devel, get acks from relevant people, links to agreed userspace changes, etc.
The ABI design here was designed and agreed upon years ago, during early gen11 development. The ABI design is that the kernel driver will faithfully initialize the hardware with the pre-determined set of magic numbers documented by the hardware team. Since these are well-documented and unchanging numbers per-platform, there's no ambiguity for userspace, firmware, etc. about what a specific mocs index means, and no need to provide additional ABI for userspace to query what the kernel used in each row or anything like that. The specific magic numbers are also ABI in the sense that we can't change the set of defined values once they're set for a platform (and it's been a long road to get the hardware and other OS software teams to understand and agree to this requirement), but we don't get to define or overrule what the initial values and order of those magic numbers are.
And this apporach has clearly not worked considering userspace and kernel have not agreed on what the abi is. We need to do better.
Userspace and kernel *do* agree on what the ABI is. There may be a delay in userspace enabling the MOCS properly on a new platform.
The fact that Mesa never started to use the MOCS entry supposedly meant for display stuff is evidence to the contrary.
I think this is just an oversight by the Mesa team where they didn't notice the gen12 table was different from gen11. I don't think it's because they don't agree that the table described in the bspec provides the right settings.
But the changes here (which make everything fully cached by default) will help catch those cases where a bit of enablement was forgotten on the UMD side. They won't "accidentally work" on initial bringup on the new platform, and the lack of proper MOCS programming will be quickly identified.
I'm looking at the history and not really convinced.
We don't have history to compare to yet because we screwed up on TGL/RKL and the use of an outdated MOCS entry by Mesa went unnoticed. If we actually do things right on ADL from day 1, then this will be our chance to catch the mistakes like this.
What is a bit vague in the formal documentation is what should be done about the reserved/dontuse table entries. In theory it wouldn't matter since they'd never be used anyway, but in reality userspace can still use them by accident, such as by forgetting to update their MOCS selection logic from past platforms (e.g., still trying to use row #1 on platforms where it isn't defined). Given that it's legal for entries to be added to MOCS tables, but never removed/modified, it follows that we should always initialize the undefined entries to fully cached; if a MOCS table update happens in the future and new rows show up, they can only become more coherent, and any userspace software that was incorrectly trying to use them previously will remain functionally correct.
What you're proposing would be a change to existing ABI --- instead of following the agreed upon contract, i915 would start defining its own set of magic numbers that potentially contradict the documentation that every other team is depending on. We already made this mistake on TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever becomes a formally defined setting in the future, the rest of the software stack will need to explicitly work around i915's bug since we can't fix it now without breaking ABI.
If you really want to redefine how the MOCS ABI works and have i915 no longer follow the current contract, I think you need to do the following:
I want the abi to be actually defined properly, and some assurance that all the stakeholders implement it correctly. Following the proper abi rules for kernel development would guarantee that.
This is what we have today. As I said, if you want to propose a change to the ABI, then you need to take the proper steps to do so. But as far as I can tell, most of your concern here boils down to not realizing that the hardware capabilities you want to use were taken away from us in gen12.
The fact that it's not actually properly documented anywhere certainly doesn't help in figuring out what is going on.
There's a pretty lengthy comment explaining everything right before the tables start in intel_mocs.c already. And the table values themselves are well-documented in the bspec (although justification for why things like PTE capability have disappeared in gen12 hardware design is something only the hardware teams know). We can extend the code comment if you think something is still lacking from the description.
I would also like if the abi can give us the best performance rather than potentially crippling it. Ie. I would expect to have a PTE MOCS setting for potential scanout buffers, or some proof that pure UC will in fact be a better choice.
Again, this isn't a choice. This is a fact of how the hardware works on gen12. Maybe you'll regain PTE flags on a future platform, but that's simply not something software can ask for on today's platforms.
There should be clear guidelines what MOCS index userspace is supposed to use in each case. That more or less used to be the case with the handful of MOCS entries we had defined (always uncached vs. always cached vs. potential scanout buffers which could be either depending on the situation). But that nice simple approach wasn't good enough for whatever reason.
I think the reality is that the real world doesn't fit into those small handful of nice categories. Userspace works with lots of different buffers, many of which are only ever used by a single process and in relation to a single IP block, and the userspace teams are really the experts on the precise cache behavior they need. Having the full table of combinations available lets them choose exactly what's necessary instead of being artificially limited to the small number i915 bothered to implement; that would be a case where we'd potentially be leaving performance on the table. I suspect that you're right that there's a relatively small subset of the MOCS table that's useful for the subset of buffers that may be shared between processes and/or between high-level graphics/media/display IP blocks, but again the userspace teams are really the experts in this area and shouldn't have trouble identifying which combinations of settings are suitable and optimal.
Matt
-- Ville Syrjälä Intel
On Thu, Sep 09, 2021 at 01:33:23PM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 10:59:36PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 11:14:15AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 08:42:15PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote: > On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote: > > On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote: > > > On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote: > > > > On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote: > > > > > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote: > > > > > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote: > > > > > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote: > > > > > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC. > > > > > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB, > > > > > > > > So adding a new MOCS table for other gen 12 devices eg. ADL. > > > > > > > > > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values") > > > > > > > > Cc: Matt Roper matthew.d.roper@intel.com > > > > > > > > Signed-off-by: Ayaz A Siddiqui ayaz.siddiqui@intel.com > > > > > > > > > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for > > > > > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup > > > > > > > for MOCS[1]. The new table is the same as the TGL table, just with > > > > > > > I915_MOCS_PTE (1) removed. > > > > > > > > > > > > And just how are people planning on handling display cacheability > > > > > > control without a PTE MOCS entry? Is Mesa/etc. already making all > > > > > > external bos uncached on these platforms just in case we might > > > > > > scan out said bo? > > > > > > > > > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12 > > > > > platforms (despite the old #define, it's not actually related to PTE, > > > > > display, etc. anymore). > > > > > > > > So can someone finally explain to me how we're supposed to cache > > > > anything that might become a scanout buffer later (eg. window system > > > > buffers)? Or are we just making everything like that UC now, and is > > > > everyone happy with that? Is userspace actually following that? > > > > > > Table entry #1 has never had anything to do with scanout on gen12+. I > > > would assume that UMDs are either using the display entry in the MOCS > > > table (which is 61 on gen12+) or some other UC entry. > > > > If 61 is meant to to be the new PTE entry wy hasn't it been defines as > > such in the code? And I know for a fact that userspace (Mesa) is not > > There is no "PTE" entry anymore. But 61 is already documented as > "displayable" in both the spec and the code: > > /* HW Special Case (Displayable) */ > MOCS_ENTRY(61,
Why is it called a "HW special case"? I don't think there's any hw magic in there?
And why aren't we setting it to PTE to get some cacheability for window back buffers and such?
Who is "we" here?
We who care about the performance of the system.
The MOCS table is a pre-defined set of per-platform magic numbers. The software teams don't get to decide what the values are, we just program the hardware with the per-platform numbers that have been agreed upon as part of a platform-wide stack (everything from low-level firmware to high level userspace should be working from the same table, defined in the bspec).
The magic numbers must be based on something. If that something is purely Windows behaviour/performance then we might be shooting ourselves in the foot here.
That's not how MOCS works. The MOCS tables define every meaningful combination of settings somewhere in the table. The *types* of settings that can be expressed change from platform to platform (e.g., "PAGETABLE" setting simply doesn't exist anymore hardware-wise) so the tables themselves differ between platforms and you may need to use different indices to get the same behavior between platforms. But if you're actually paying attention to the tables and choosing the right entries, you're not going to leave any performance on the table.
Once we know what the per-platform magic numbers are, we're supposed to pick the table entry that matches the behavior we're trying to accomplish. If you want some specific level of cacheability, then you select a table row that gives you that. Maybe 61 isn't the best setting, I don't know; userspace can pick whichever defined setting is actually best, using the data from the table. But table row #1 is already well-documented as reserved/dontuse across the full stack; the fact that row #1 had values similar to PTE on Icelake hardware doesn't carry forward to any post-gen11 platform.
The only way you can get LLC cacheability for an external BO (window back buffers and such) is by using a MOCS entry that directs the hardware to consult the PTEs. Otherwise the client doing the rendering would have to know ahead of time whether the buffer is going to be directly scanned out by the compositor or not, for which there is no protocol in X or wayland.
It sounds like you're complaining about gen12 hardware design rather than anything actually software or ABI related. 'Consult the PTEs' hasn't been something the hardware has supported since gen11. We still have macros with PTE or PAGETABLE in their names, but those aren't accurate descriptions of what they do on current platforms.
I've heard this a few times, but so far I've not been able to find any evidence for it in bspec. I suppose to find out for sure I'm going to have to test it on actual hardware.
For gen11, bspec page 560 describes the layout of the GFX_MOCS_x registers (which at the time were per-engine). Specifically:
Memory type information used in LLC/eDRAM. 00b: Use Cacheability Controls from page table / UC with Fence (if coherent cycle). 01b: Uncacheable (UC) – non-cacheable. 10b: Writethrough (WT). 11b: Writeback (WB).So on that platform you can get the PTE behavior you're after by programming a value of 00 into the relevant bits (1:0).
For gen12+, bspec page 44053 describes the layout of GLOB_MOCS_LECC_x (which is now a global value, not per-engine):
Memory type information used in LLC/eDRAM.
00: Uncacheable (UC) 01: Uncacheable (WC) 10: Writethrough (WT) 11: Writeback (WB)
So the value of '00' as expressed in a MOCS table no longer means what it did on older platforms. There's also a "Description" field on this page that shows that the old meaning of 00 is explicitly removed by gen12 design.
Page 45100 still has the old description for tgl. Sigh.
I did stumble on some future platform stuff that shuffles around the MOCS stuff once again, and looks like it adds in a specific PTE vs. MOCS bit. So if it was removed, it's almost immediately coming back.
Yes, bspec page 65553 makes it look like there is a new bit to handle this eventually on a future platform. But we have multiple new platforms between now and then that we still need to support properly, starting with ADL, and that bit (and the rest of the various MOCS changes) aren't something we can use yet.
We note this in the code:
/* * Note: LE_0_PAGETABLE works only up to Gen11; for newer gens it means * the same as LE_UC */
Even on gen11 the PAT entry was documented as being deprecated, so we knew it was going away. I don't know why the hardware design changed, but it did.
Yeah, I've seen it, but could never figure out on what it was based.
Historically I believe LLC cacheability has been on average a win. Some workloads can do better with UC though. So if we are giving up on LLC cacheability we should have some numbers to back up that decision so that we're not dropping tons of performance on the floor.
It's not our decision to make. The hardware design has already made this decision for us. No matter what we program into the MOCS tables; we're simply not getting back the behavior you're asking for and blindly using the "LE_0_PAGETABLE" flags or whatever is just giving you uncached.
As said it looks to be coming back soon, if it ever went away.
> > using entry 61. I think there is a massive communication gap here > > where everyone just seems to assume the other side is doing something. > > > > Could someone actually come up with a clear abi definition for this > > and get all the stakeholders to sign off on it? > > The agreement between the i915 team, various userspace teams, Windows > driver team, hardware architects, software architects, and bspec writers > was just completed; that's what triggered the kernel updates here (and > I'm guessing is triggering similar work on the UMD side). It's also why > we held off on removing the force_probe flag on ADL until now since we > couldn't consider enablement of the platform complete until the > agreement and definitions here was finalized.
Can we get that agreement visible on the mailing list? Since MOCS is abi I don't see why we shouldn't follow the normal abi rules for these, ie. post to dri-devel, get acks from relevant people, links to agreed userspace changes, etc.
The ABI design here was designed and agreed upon years ago, during early gen11 development. The ABI design is that the kernel driver will faithfully initialize the hardware with the pre-determined set of magic numbers documented by the hardware team. Since these are well-documented and unchanging numbers per-platform, there's no ambiguity for userspace, firmware, etc. about what a specific mocs index means, and no need to provide additional ABI for userspace to query what the kernel used in each row or anything like that. The specific magic numbers are also ABI in the sense that we can't change the set of defined values once they're set for a platform (and it's been a long road to get the hardware and other OS software teams to understand and agree to this requirement), but we don't get to define or overrule what the initial values and order of those magic numbers are.
And this apporach has clearly not worked considering userspace and kernel have not agreed on what the abi is. We need to do better.
Userspace and kernel *do* agree on what the ABI is. There may be a delay in userspace enabling the MOCS properly on a new platform.
The fact that Mesa never started to use the MOCS entry supposedly meant for display stuff is evidence to the contrary.
I think this is just an oversight by the Mesa team where they didn't notice the gen12 table was different from gen11. I don't think it's because they don't agree that the table described in the bspec provides the right settings.
I never suggested they didn't agree. I'm saying that because the documentation/communication is poor these kinds of oversights just tend to happen. That's why some real abi docs explaining which entry one should use would be good. And following the usual abi rules of having links to corresponding userspace changes would guarantee that userspace is in sync before we lock things down.
But the changes here (which make everything fully cached by default) will help catch those cases where a bit of enablement was forgotten on the UMD side. They won't "accidentally work" on initial bringup on the new platform, and the lack of proper MOCS programming will be quickly identified.
I'm looking at the history and not really convinced.
We don't have history to compare to yet because we screwed up on TGL/RKL and the use of an outdated MOCS entry by Mesa went unnoticed. If we actually do things right on ADL from day 1, then this will be our chance to catch the mistakes like this.
tgl wasn't the only time when our MOCS stuff was unclear/used wrong. In fact the L3 cacheability vs. display question is still open and I never managed to get a straight answer what userspace wants. So for the time being we make scanout buffers L3 cacheable on most platforms, which means at least theoretically evictions from L3 into LLC can cause display corruption.
And on a few platforms we don't use L3 for display buffers mostly by accident. CHV being one case where I think L3 caching would actually be safe since there's no LLC to mess up the display coherency.
We also didn't even have the proper MOCS entries defined for L3 evictions/etc. until recently when I+Chris noticed that this stuff is just totally missing.
What is a bit vague in the formal documentation is what should be done about the reserved/dontuse table entries. In theory it wouldn't matter since they'd never be used anyway, but in reality userspace can still use them by accident, such as by forgetting to update their MOCS selection logic from past platforms (e.g., still trying to use row #1 on platforms where it isn't defined). Given that it's legal for entries to be added to MOCS tables, but never removed/modified, it follows that we should always initialize the undefined entries to fully cached; if a MOCS table update happens in the future and new rows show up, they can only become more coherent, and any userspace software that was incorrectly trying to use them previously will remain functionally correct.
What you're proposing would be a change to existing ABI --- instead of following the agreed upon contract, i915 would start defining its own set of magic numbers that potentially contradict the documentation that every other team is depending on. We already made this mistake on TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever becomes a formally defined setting in the future, the rest of the software stack will need to explicitly work around i915's bug since we can't fix it now without breaking ABI.
If you really want to redefine how the MOCS ABI works and have i915 no longer follow the current contract, I think you need to do the following:
I want the abi to be actually defined properly, and some assurance that all the stakeholders implement it correctly. Following the proper abi rules for kernel development would guarantee that.
This is what we have today. As I said, if you want to propose a change to the ABI, then you need to take the proper steps to do so. But as far as I can tell, most of your concern here boils down to not realizing that the hardware capabilities you want to use were taken away from us in gen12.
The fact that it's not actually properly documented anywhere certainly doesn't help in figuring out what is going on.
There's a pretty lengthy comment explaining everything right before the tables start in intel_mocs.c already. And the table values themselves are well-documented in the bspec (although justification for why things like PTE capability have disappeared in gen12 hardware design is something only the hardware teams know). We can extend the code comment if you think something is still lacking from the description.
Proper abi docs should probably be somewhere a bit more prominent. Maybe we should even have real defines for these in some uapi header.
I would also like if the abi can give us the best performance rather than potentially crippling it. Ie. I would expect to have a PTE MOCS setting for potential scanout buffers, or some proof that pure UC will in fact be a better choice.
Again, this isn't a choice. This is a fact of how the hardware works on gen12. Maybe you'll regain PTE flags on a future platform, but that's simply not something software can ask for on today's platforms.
There should be clear guidelines what MOCS index userspace is supposed to use in each case. That more or less used to be the case with the handful of MOCS entries we had defined (always uncached vs. always cached vs. potential scanout buffers which could be either depending on the situation). But that nice simple approach wasn't good enough for whatever reason.
I think the reality is that the real world doesn't fit into those small handful of nice categories. Userspace works with lots of different buffers, many of which are only ever used by a single process and in relation to a single IP block, and the userspace teams are really the experts on the precise cache behavior they need.
Well, Mesa just uses just two entries. Maybe three these days (iirc there was some magic index thing for some extra data cache thing). I'm sure you could no end of very workload/resolution specific tuning. Maybe the Windows people do a lot of that but Mesa certainly doesn't.
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Matt Roper -
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