> -----Original Message----- > From: Alex Deucher <alexdeucher@xxxxxxxxx> > Sent: Tuesday, August 24, 2021 12:44 PM > To: Dave Airlie <airlied@xxxxxxxxx> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>; Jason Gunthorpe <jgg@xxxxxxxx>; Christian König <christian.koenig@xxxxxxx>; Gal Pressman > <galpress@xxxxxxxxxx>; Daniel Vetter <daniel@xxxxxxxx>; Sumit Semwal <sumit.semwal@xxxxxxxxxx>; Doug Ledford > <dledford@xxxxxxxxxx>; open list:DMA BUFFER SHARING FRAMEWORK <linux-media@xxxxxxxxxxxxxxx>; dri-devel <dri- > devel@xxxxxxxxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>; linux-rdma <linux-rdma@xxxxxxxxxxxxxxx>; Gabbay, > Oded (Habana) <ogabbay@xxxxxxxxx>; Tayar, Tomer (Habana) <ttayar@xxxxxxxxx>; Yossi Leybovich <sleybo@xxxxxxxxxx>; Alexander > Matushevsky <matua@xxxxxxxxxx>; Leon Romanovsky <leonro@xxxxxxxxxx>; Xiong, Jianxin <jianxin.xiong@xxxxxxxxx> > Subject: Re: [RFC] Make use of non-dynamic dmabuf in RDMA > > 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 > In addition, the dma-buf exporter can be a non-GPU device. Jianxin > > > > 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.
