Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

[Rob Clark <robdclark@gmail.com>]

On Wed, Sep 7, 2022 at 3:25 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> On 8/23/22 19:47, Rob Clark wrote:
> > On Tue, Aug 23, 2022 at 3:01 AM Christian König
> > <ckoenig.leichtzumerken@gmail.com> wrote:
> >>
> >> Am 22.08.22 um 19:26 schrieb Dmitry Osipenko:
> >>> On 8/16/22 22:55, Dmitry Osipenko wrote:
> >>>> On 8/16/22 15:03, Christian König wrote:
> >>>>> Am 16.08.22 um 13:44 schrieb Dmitry Osipenko:
> >>>>>> [SNIP]
> >>>>>>> The other complication I noticed is that we don't seem to keep around
> >>>>>>> the fd after importing to a GEM handle.  And I could imagine that
> >>>>>>> doing so could cause issues with too many fd's.  So I guess the best
> >>>>>>> thing is to keep the status quo and let drivers that cannot mmap
> >>>>>>> imported buffers just fail mmap?
> >>>>>> That actually should be all the drivers excluding those that use
> >>>>>> DRM-SHMEM because only DRM-SHMEM uses dma_buf_mmap(), that's why it
> >>>>>> works for Panfrost. I'm pretty sure mmaping of imported GEMs doesn't
> >>>>>> work for the MSM driver, isn't it?
> >>>>>>
> >>>>>> Intel and AMD drivers don't allow to map the imported dma-bufs. Both
> >>>>>> refuse to do the mapping.
> >>>>>>
> >>>>>> Although, AMDGPU "succeeds" to do the mapping using
> >>>>>> AMDGPU_GEM_DOMAIN_GTT, but then touching the mapping causes bus fault,
> >>>>>> hence mapping actually fails. I think it might be the AMDGPU
> >>>>>> driver/libdrm bug, haven't checked yet.
> >>>>> That's then certainly broken somehow. Amdgpu should nerve ever have
> >>>>> allowed to mmap() imported DMA-bufs and the last time I check it didn't.
> >>>> I'll take a closer look. So far I can only tell that it's a kernel
> >>>> driver issue because once I re-applied this "Don't map imported GEMs"
> >>>> patch, AMDGPU began to refuse mapping AMDGPU_GEM_DOMAIN_GTT.
> >>>>
> >>>>>> So we're back to the point that neither of DRM drivers need to map
> >>>>>> imported dma-bufs and this was never tested. In this case this patch is
> >>>>>> valid, IMO.
> >>>> Actually, I'm now looking at Etnaviv and Nouveau and seems they should
> >>>> map imported dma-buf properly. I know that people ran Android on
> >>>> Etnaviv. So maybe devices with a separated GPU/display need to map
> >>>> imported display BO for Android support. Wish somebody who ran Android
> >>>> on one of these devices using upstream drivers could give a definitive
> >>>> answer. I may try to test Nouveau later on.
> >>>>
> >>> Nouveau+Intel combo doesn't work because of [1] that says:
> >>>
> >>> "Refuse to fault imported pages. This should be handled (if at all) by
> >>> redirecting mmap to the exporter."
> >>>
> >>> [1]
> >>> https://elixir.bootlin.com/linux/v5.19/source/drivers/gpu/drm/ttm/ttm_bo_vm.c#L154
> >>>
> >>> Interestingly, I noticed that there are IGT tests which check prime
> >>> mmaping of Nouveau+Intel [2] (added 9 years ago), but they fail as well,
> >>> as expected. The fact that IGT has such tests is interesting because it
> >>> suggests that the mapping worked in the past. It's also surprising that
> >>> nobody cared to fix the failing tests. For the reference, I checked
> >>> v5.18 and today's linux-next.
> >>>
> >>> [2]
> >>> https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/prime_nv_test.c#L132
> >>>
> >>> Starting subtest: nv_write_i915_cpu_mmap_read
> >>> Received signal SIGBUS.
> >>> Stack trace:
> >>>   #0 [fatal_sig_handler+0x163]
> >>>   #1 [__sigaction+0x50]
> >>>   #2 [__igt_unique____real_main354+0x406]
> >>>   #3 [main+0x23]
> >>>   #4 [__libc_start_call_main+0x80]
> >>>   #5 [__libc_start_main+0x89]
> >>>   #6 [_start+0x25]
> >>> Subtest nv_write_i915_cpu_mmap_read: CRASH (0,005s)
> >>>
> >>> Starting subtest: nv_write_i915_gtt_mmap_read
> >>> Received signal SIGBUS.
> >>> Stack trace:
> >>>   #0 [fatal_sig_handler+0x163]
> >>>   #1 [__sigaction+0x50]
> >>>   #2 [__igt_unique____real_main354+0x33d]
> >>>   #3 [main+0x23]
> >>>   #4 [__libc_start_call_main+0x80]
> >>>   #5 [__libc_start_main+0x89]
> >>>   #6 [_start+0x25]
> >>> Subtest nv_write_i915_gtt_mmap_read: CRASH (0,004s)
> >>>
> >>> I'm curious about the Etnaviv driver because it uses own shmem
> >>> implementation and maybe it has a working mmaping of imported GEMs since
> >>> it imports the dma-buf pages into Entaviv BO. Although, it should be
> >>> risking to map pages using a different caching attributes (WC) from the
> >>> exporter, which is prohibited on ARM ad then one may try to map imported
> >>> udmabuf.
>
> I see now that Etnaviv uses dma_buf_mmap(), so it should be okay.
>
> >>> Apparently, the Intel DG TTM driver should be able to map imported
> >>> dma-buf because it sets TTM_TT_FLAG_EXTERNAL_MAPPABLE.
> >>
> >> Even with that flag set it is illegal to map the pages directly by an
> >> importer.
> >>
> >> If that ever worked then the only real solution is to redirect mmap()
> >> calls on importer BOs to dma_buf_mmap().
> >
> > Yeah, I think this is the best option.  Forcing userspace to hang on
> > to the fd just in case someone calls readpix would be pretty harsh.
>
> Actually, I proposed this couple months ago [1].
>
> [1] https://patchwork.freedesktop.org/patch/487481/
>
> What's not clear to me is how userspace is supposed to sync CPU accesses
> for imported GEMs. Either userspace need to use dma_buf_sync UAPI for
> dmabuf of imported GEM or importer driver should do that, or there is
> some other option?

For anything involving the GPU, userspace already needs to sync access
(ie. GPUs are asynchronous).. this is done using drm_gem_object::resv
(which comes from dma_buf::resv)..

Possibly we should require that
dma_buf_begin_cpu_access()/dma_buf_end_cpu_access().. but in practice
if the gl/vk driver needs CPU access it is to read the results of it's
own GPU rendering.  Disallow that and all of deqp/piglit/etc will
break on systems where "window" surfaces are dma-buf imports (like
android), which would be a rather unpopular outcome ;-)

BR,
-R