Hey Daniel, just a ping on a bunch of questions i posted bellow.
Andtey ________________________________ From: Grodzovsky, Andrey Andrey.Grodzovsky@amd.com Sent: 25 November 2020 14:34 To: Daniel Vetter daniel@ffwll.ch; Koenig, Christian Christian.Koenig@amd.com Cc: robh@kernel.org robh@kernel.org; daniel.vetter@ffwll.ch daniel.vetter@ffwll.ch; dri-devel@lists.freedesktop.org dri-devel@lists.freedesktop.org; eric@anholt.net eric@anholt.net; ppaalanen@gmail.com ppaalanen@gmail.com; amd-gfx@lists.freedesktop.org amd-gfx@lists.freedesktop.org; gregkh@linuxfoundation.org gregkh@linuxfoundation.org; Deucher, Alexander Alexander.Deucher@amd.com; l.stach@pengutronix.de l.stach@pengutronix.de; Wentland, Harry Harry.Wentland@amd.com; yuq825@gmail.com yuq825@gmail.com Subject: Re: [PATCH v3 05/12] drm/ttm: Expose ttm_tt_unpopulate for driver use
On 11/25/20 11:36 AM, Daniel Vetter wrote:
On Wed, Nov 25, 2020 at 01:57:40PM +0100, Christian König wrote:
Am 25.11.20 um 11:40 schrieb Daniel Vetter:
On Tue, Nov 24, 2020 at 05:44:07PM +0100, Christian König wrote:
Am 24.11.20 um 17:22 schrieb Andrey Grodzovsky:
On 11/24/20 2:41 AM, Christian König wrote:
Am 23.11.20 um 22:08 schrieb Andrey Grodzovsky: > On 11/23/20 3:41 PM, Christian König wrote: >> Am 23.11.20 um 21:38 schrieb Andrey Grodzovsky: >>> On 11/23/20 3:20 PM, Christian König wrote: >>>> Am 23.11.20 um 21:05 schrieb Andrey Grodzovsky: >>>>> On 11/25/20 5:42 AM, Christian König wrote: >>>>>> Am 21.11.20 um 06:21 schrieb Andrey Grodzovsky: >>>>>>> It's needed to drop iommu backed pages on device unplug >>>>>>> before device's IOMMU group is released. >>>>>> It would be cleaner if we could do the whole >>>>>> handling in TTM. I also need to double check >>>>>> what you are doing with this function. >>>>>> >>>>>> Christian. >>>>> Check patch "drm/amdgpu: Register IOMMU topology >>>>> notifier per device." to see >>>>> how i use it. I don't see why this should go >>>>> into TTM mid-layer - the stuff I do inside >>>>> is vendor specific and also I don't think TTM is >>>>> explicitly aware of IOMMU ? >>>>> Do you mean you prefer the IOMMU notifier to be >>>>> registered from within TTM >>>>> and then use a hook to call into vendor specific handler ? >>>> No, that is really vendor specific. >>>> >>>> What I meant is to have a function like >>>> ttm_resource_manager_evict_all() which you only need >>>> to call and all tt objects are unpopulated. >>> So instead of this BO list i create and later iterate in >>> amdgpu from the IOMMU patch you just want to do it >>> within >>> TTM with a single function ? Makes much more sense. >> Yes, exactly. >> >> The list_empty() checks we have in TTM for the LRU are >> actually not the best idea, we should now check the >> pin_count instead. This way we could also have a list of the >> pinned BOs in TTM. > So from my IOMMU topology handler I will iterate the TTM LRU for > the unpinned BOs and this new function for the pinned ones ? > It's probably a good idea to combine both iterations into this > new function to cover all the BOs allocated on the device. Yes, that's what I had in my mind as well.
>> BTW: Have you thought about what happens when we unpopulate >> a BO while we still try to use a kernel mapping for it? That >> could have unforeseen consequences. > Are you asking what happens to kmap or vmap style mapped CPU > accesses once we drop all the DMA backing pages for a particular > BO ? Because for user mappings > (mmap) we took care of this with dummy page reroute but indeed > nothing was done for in kernel CPU mappings. Yes exactly that.
In other words what happens if we free the ring buffer while the kernel still writes to it?
Christian.
While we can't control user application accesses to the mapped buffers explicitly and hence we use page fault rerouting I am thinking that in this case we may be able to sprinkle drm_dev_enter/exit in any such sensitive place were we might CPU access a DMA buffer from the kernel ?
Yes, I fear we are going to need that.
Uh ... problem is that dma_buf_vmap are usually permanent things. Maybe we could stuff this into begin/end_cpu_access
Do you mean guarding with drm_dev_enter/exit in dma_buf_ops.begin/end_cpu_access driver specific hook ?
(but only for the kernel, so a bit tricky)?
Why only kernel ? Why is it a problem to do it if it comes from dma_buf_ioctl by some user process ? And if we do need this distinction I think we should be able to differentiate by looking at current->mm (i.e. mm_struct) pointer being NULL for kernel thread.
Oh very very good point! I haven't thought about DMA-buf mmaps in this context yet.
btw the other issue with dma-buf (and even worse with dma_fence) is refcounting of the underlying drm_device. I'd expect that all your callbacks go boom if the dma_buf outlives your drm_device. That part isn't yet solved in your series here.
Well thinking more about this, it seems to be a another really good argument why mapping pages from DMA-bufs into application address space directly is a very bad idea :)
But yes, we essentially can't remove the device as long as there is a DMA-buf with mappings. No idea how to clean that one up.
drm_dev_get/put in drm_prime helpers should get us like 90% there I think.
What are the other 10% ?
The even more worrying thing is random dma_fence attached to the dma_resv object. We could try to clean all of ours up, but they could have escaped already into some other driver. And since we're talking about egpu hotunplug, dma_fence escaping to the igpu is a pretty reasonable use-case.
I have no how to fix that one :-/ -Daniel
I assume you are referring to sync_file_create/sync_file_get_fence API for dma_fence export/import ? So with DMA bufs we have the drm_gem_object as exporter specific private data and so we can do drm_dev_get and put at the drm_gem_object layer to bind device life cycle to that of each GEM object but, we don't have such mid-layer for dma_fence which could allow us to increment device reference for each fence out there related to that device - is my understanding correct ?
Andrey
Andrey
Christian.
-Daniel
Things like CPU page table updates, ring buffer accesses and FW memcpy ? Is there other places ?
Puh, good question. I have no idea.
Another point is that at this point the driver shouldn't access any such buffers as we are at the process finishing the device. AFAIK there is no page fault mechanism for kernel mappings so I don't think there is anything else to do ?
Well there is a page fault handler for kernel mappings, but that one just prints the stack trace into the system log and calls BUG(); :)
Long story short we need to avoid any access to released pages after unplug. No matter if it's from the kernel or userspace.
Regards, Christian.
Andrey