On Thu, May 7, 2020 at 10:44 AM Chris Wilson chris@chris-wilson.co.uk wrote:
Quoting Jason Ekstrand (2020-05-07 16:36:00)
The Vulkan driver in Mesa for Intel hardware never uses relocations if it's running on a version of i915 that supports at least softpin which all versions of i915 supporting Gen12 do. On the OpenGL side, Gen12 is only supported by iris which never uses relocations. The older i965 driver in Mesa does use relocations but it only supports Intel hardware through Gen11 and has been deprecated for all hardware Gen9+. The entire relocation UAPI and related infrastructure, therefore, doesn't have any open-source userspace consumer starting with Gen12.
Rejecting relocations starting with Gen12 has the benefit that we don't have to bother supporting it on platforms with local memory. Given how much CPU touching of memory is required for relocations, not having to do so on platforms where not all memory is directly CPU-accessible carries significant advantages.
You are not supplying them, the kernel is not checking them [as they don't exist], so there is no material benefit. The only question is maintainability.
How confident are you that you will never use them
Confident enough to send this patch. Especially in a Vulkan world where it's very hard to tell which bits of memory are actually in-use on the GPU, stalling to relocate is performance death. With a 48-bit GTT, there's no need to have the kernel involved in address space assignment so relocations don't really serve much purpose. We did potentially have one use for them with VK_KHR_performance_query but we're going out of our way to avoid them there. If we desperately need relocations, we can do them from userspace.
and rewrite the media-driver?
I'm pretty sure they're working on getting rid of them. I'm checking on that right now.
The code exists, will be tested, and can just as easily expire with the rest of execbuffer2.
Sure. The question, again, is maintenance. If we're spending piles of time trying to figure out how to keep relocations going in a local memory world, that's likely a waste. Relocations can sit and rot on Gen11 and below until we finally make execbuffer3 a reality and then they can rot in the deprecated execbuffer2 ioct.
There is a bit of a question here about what to do with IGT. I suspect it uses relocations for a lot of stuff and the fallout there could be significant. (I explicitly made this patch so it won't actually build because I don't hate our CI people.)
--Jason