On Tue, Aug 24, 2021 at 3:16 PM Dave Airlie <airlied@xxxxxxxxx> wrote: > > On Wed, 25 Aug 2021 at 03:36, John Hubbard <jhubbard@xxxxxxxxxx> wrote: > > > > On 8/24/21 10:32 AM, Jason Gunthorpe wrote: > > ... > > >>> And yes at least for the amdgpu driver we migrate the memory to host > > >>> memory as soon as it is pinned and I would expect that other GPU drivers > > >>> do something similar. > > >> > > >> Well...for many topologies, migrating to host memory will result in a > > >> dramatically slower p2p setup. For that reason, some GPU drivers may > > >> want to allow pinning of video memory in some situations. > > >> > > >> Ideally, you've got modern ODP devices and you don't even need to pin. > > >> But if not, and you still hope to do high performance p2p between a GPU > > >> and a non-ODP Infiniband device, then you would need to leave the pinned > > >> memory in vidmem. > > >> > > >> So I think we don't want to rule out that behavior, right? Or is the > > >> thinking more like, "you're lucky that this old non-ODP setup works at > > >> all, and we'll make it work by routing through host/cpu memory, but it > > >> will be slow"? > > > > > > I think it depends on the user, if the user creates memory which is > > > permanently located on the GPU then it should be pinnable in this way > > > without force migration. But if the memory is inherently migratable > > > then it just cannot be pinned in the GPU at all as we can't > > > indefinately block migration from happening eg if the CPU touches it > > > later or something. > > > > > > > OK. I just want to avoid creating any API-level assumptions that dma_buf_pin() > > necessarily implies or requires migrating to host memory. > > I'm not sure we should be allowing dma_buf_pin at all on > non-migratable memory, what's to stop someone just pinning all the > VRAM and making the GPU unuseable? In a lot of cases we have GPUs with more VRAM than system memory, but we allow pinning in system memory. Alex > > I understand not considering more than a single user in these > situations is enterprise thinking, but I do worry about pinning is > always fine type of thinking when things are shared or multi-user. > > My impression from this is we've designed hardware that didn't > consider the problem, and now to let us use that hardware in horrible > ways we should just allow it to pin all the things. > > Dave.
