On 01/09/2021 14:24, Christian König wrote: > > > Am 01.09.21 um 13:20 schrieb Gal Pressman: >> 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? > > I think so, yes. > >> If so, do we still want a move_notify callback for non-dynamic importers? A noop? > > Well we could make the move_notify callback optional, e.g. so that you get the > new locking approach but still pin the buffers manually with dma_buf_pin(). Thanks Christian! So the end result will look similar to the original patch I posted, where peer2peer can be enabled without providing move_notify, correct?
