On 24/08/2021 20:32, Jason Gunthorpe wrote: > On Tue, Aug 24, 2021 at 10:27:23AM -0700, John Hubbard wrote: >> On 8/24/21 2:32 AM, Christian König wrote: >>> Am 24.08.21 um 11:06 schrieb Gal Pressman: >>>> On 23/08/2021 13:43, Christian König wrote: >>>>> Am 21.08.21 um 11:16 schrieb Gal Pressman: >>>>>> On 20/08/2021 17:32, Jason Gunthorpe wrote: >>>>>>> On Fri, Aug 20, 2021 at 03:58:33PM +0300, Gal Pressman wrote: >> ... >>>>>> IIUC, we're talking about three different exporter "types": >>>>>> - Dynamic with move_notify (requires ODP) >>>>>> - Dynamic with revoke_notify >>>>>> - Static >>>>>> >>>>>> Which changes do we need to make the third one work? >>>>> Basically none at all in the framework. >>>>> >>>>> You just need to properly use the dma_buf_pin() function when you start using a >>>>> buffer (e.g. before you create an attachment) and the dma_buf_unpin() function >>>>> after you are done with the DMA-buf. >>>> I replied to your previous mail, but I'll ask again. >>>> Doesn't the pin operation migrate the memory to host memory? >>> >>> Sorry missed your previous reply. >>> >>> 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. So are we OK with exporters implementing dma_buf_pin() without migrating the memory? If so, do we still want a move_notify callback for non-dynamic importers? A noop?
