On Thu, Apr 16, 2015 at 3:55 PM, Terje Bergstrom <[email protected]> wrote: > > On 04/16/2015 12:31 PM, Ilia Mirkin wrote: >> >> Two questions -- >> >> (a) What's the perf impact of doing this? Less work for the GPU MMU >> but more work for the IOMMU... >> (b) Would it be a good idea to do this for desktop GPUs that are on >> CPUs with IOMMUs in them (VT-d and whatever the AMD one is)? Is there >> some sort of shared API for this stuff that you should be (or are?) >> using? > > a) Using IOMMU mapping is the best way of getting contiguous post-GMMU > address spaces. The continuity is required to be able to use frame buffer > compression. So overall performance impact when compression is factored in > is about 20-30%. > > If compression is left out of the equation, the impact SMMU translation and > small versus large pages should not be noticeable, but I haven't measured > it. We have measured large versus small pages with compression disabled in > both cases in gk20a and the difference was noise.
Ah, I never made the connection to compression. I had assumed it was something done at a higher level by PGRAPH rather than at the PTE level by the VM. [I did know that you had to set compression at the PTE level, but didn't think that page size mattered.] > > Additional advantage is extra protection against GPU accidentally walking > over kernel memory if kernel driver has a bug. Yeah, IOMMU's are nice :) _______________________________________________ Nouveau mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/nouveau
