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

Rob Clark
2022-08-16T22:19:03Z

On Tue, Aug 16, 2022 at 4:45 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> On 8/12/22 18:01, Rob Clark wrote:
> > On Fri, Aug 12, 2022 at 7:57 AM Rob Clark <robdclark@gmail.com> wrote:
> >>
> >> On Fri, Aug 12, 2022 at 4:26 AM Dmitry Osipenko
> >> <dmitry.osipenko@collabora.com> wrote:
> >>>
> >>> On 8/11/22 02:19, Rob Clark wrote:
> >>>> On Wed, Aug 10, 2022 at 3:23 PM Dmitry Osipenko
> >>>> <dmitry.osipenko@collabora.com> wrote:
> >>>>>
> >>>>> On 8/11/22 01:03, Rob Clark wrote:
> >>>>>> On Wed, Aug 10, 2022 at 12:26 PM Dmitry Osipenko
> >>>>>> <dmitry.osipenko@collabora.com> wrote:
> >>>>>>>
> >>>>>>> On 8/10/22 18:08, Rob Clark wrote:
> >>>>>>>> On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >>>>>>>>>
> >>>>>>>>> On Wed, Jul 06, 2022 at 10:02:07AM +0300, Dmitry Osipenko wrote:
> >>>>>>>>>> On 7/6/22 00:48, Rob Clark wrote:
> >>>>>>>>>>> On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@amd.com> wrote:
> >>>>>>>>>>>>
> >>>>>>>>>>>> Am 01.07.22 um 11:02 schrieb Dmitry Osipenko:
> >>>>>>>>>>>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
> >>>>>>>>>>>>> handle imported dma-bufs properly, which results in mapping of something
> >>>>>>>>>>>>> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
> >>>>>>>>>>>>> userspace writes to the memory mapping of a dma-buf that was imported into
> >>>>>>>>>>>>> Tegra's DRM GEM.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Majority of DRM drivers prohibit mapping of the imported GEM objects.
> >>>>>>>>>>>>> Mapping of imported GEMs require special care from userspace since it
> >>>>>>>>>>>>> should sync dma-buf because mapping coherency of the exporter device may
> >>>>>>>>>>>>> not match the DRM device. Let's prohibit the mapping for all DRM drivers
> >>>>>>>>>>>>> for consistency.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> >>>>>>>>>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> >>>>>>>>>>>>
> >>>>>>>>>>>> I'm pretty sure that this is the right approach, but it's certainly more
> >>>>>>>>>>>> than possible that somebody abused this already.
> >>>>>>>>>>>
> >>>>>>>>>>> I suspect that this is abused if you run deqp cts on android.. ie. all
> >>>>>>>>>>> winsys buffers are dma-buf imports from gralloc. And then when you
> >>>>>>>>>>> hit readpix...
> >>>>>>>>>>>
> >>>>>>>>>>> You might only hit this in scenarios with separate gpu and display (or
> >>>>>>>>>>> dGPU+iGPU) because self-imports are handled differently in
> >>>>>>>>>>> drm_gem_prime_import_dev().. and maybe not in cases where you end up
> >>>>>>>>>>> with a blit from tiled/compressed to linear.. maybe that narrows the
> >>>>>>>>>>> scope enough to just fix it in userspace?
> >>>>>>>>>>
> >>>>>>>>>> Given that that only drivers which use DRM-SHMEM potentially could've
> >>>>>>>>>> map imported dma-bufs (Panfrost, Lima) and they already don't allow to
> >>>>>>>>>> do that, I think we're good.
> >>>>>>>>>
> >>>>>>>>> So can I have an ack from Rob here or are there still questions that this
> >>>>>>>>> might go boom?
> >>>>>>>>>
> >>>>>>>>> Dmitry, since you have a bunch of patches merged now I think would also be
> >>>>>>>>> good to get commit rights so you can drive this more yourself. I've asked
> >>>>>>>>> Daniel Stone to help you out with getting that.
> >>>>>>>>
> >>>>>>>> I *think* we'd be ok with this on msm, mostly just by dumb luck.
> >>>>>>>> Because the dma-buf's we import will be self-import. I'm less sure
> >>>>>>>> about panfrost (src/panfrost/lib/pan_bo.c doesn't seem to have a
> >>>>>>>> special path for imported dma-bufs either, and in that case they won't
> >>>>>>>> be self-imports.. but I guess no one has tried to run android cts on
> >>>>>>>> panfrost).
> >>>>>>>
> >>>>>>> The last time I tried to mmap dma-buf imported to Panfrost didn't work
> >>>>>>> because Panfrost didn't implement something needed for that. I'll need
> >>>>>>> to take a look again because can't recall what it was.
> >>> Upd: I re-checked Panfrost using today's linux-next and mapping of
> >>> imported dma-buf works, I mapped imported buf from video decoder.
> >>> Apparently previously I had some local kernel change that broke the mapping.
> >>>
> >>>>>>>> What about something less drastic to start, like (apologies for
> >>>>>>>> hand-edited patch):
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> >>>>>>>> index 86d670c71286..fc9ec42fa0ab 100644
> >>>>>>>> --- a/drivers/gpu/drm/drm_gem.c
> >>>>>>>> +++ b/drivers/gpu/drm/drm_gem.c
> >>>>>>>> @@ -1034,6 +1034,10 @@ int drm_gem_mmap_obj(struct drm_gem_object
> >>>>>>>> *obj, unsigned long obj_size,
> >>>>>>>> {
> >>>>>>>> int ret;
> >>>>>>>>
> >>>>>>>> + WARN_ON_ONCE(obj->import_attach);
> >>>>>>>
> >>>>>>> This will hang NVIDIA Tegra, which is what this patch fixed initially.
> >>>>>>> If neither of upstream DRM drivers need to map imported dma-bufs and
> >>>>>>> never needed, then why do we need this?
> >>>>>>
> >>>>>> oh, tegra isn't using shmem helpers? I assumed it was. Well my point
> >>>>>> was to make a more targeted fail on tegra, and a WARN_ON for everyone
> >>>>>> else to make it clear that what they are doing is undefined behavior.
> >>>>>> Because so far existing userspace (or well, panfrost and freedreno at
> >>>>>> least, those are the two I know or checked) don't make special cases
> >>>>>> for mmap'ing against the dmabuf fd against the dmabuf fd instead of
> >>>>>> the drm device fd.
> >>>>>
> >>>>> It's not clear to me what bad Android does form yours comments. Does it
> >>>>> export dma-buf from GPU and then import it to GPU? If yes, then DRM core
> >>>>> has a check for the self-importing [1].
> >>>>>
> >>>>> [1]
> >>>>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_prime.c#L918
> >>>>>
> >>>>> If you're meaning something else, then please explain in a more details.
> >>>>
> >>>> So, android/gralloc allocates buffers externally to the driver and
> >>>> imports them into driver. (And that seems to not just be window
> >>>> surfaces, but in cases random textures, etc)
> >>>>
> >>>> In the normal case these should be allocated from drm/msm so it should
> >>>> hit [1].. this is the "dumb luck" I mentioned earlier. But I'm not
> >>>> confident enough to say that there is no other case.
> >>>>
> >>>>>
> >>>>>> I *think* it should work out that we don't hit this path with
> >>>>>> freedreno but on android I can't really guarantee or prove it. So
> >>>>>> your patch would potentially break existing working userspace. Maybe
> >>>>>> it is userspace that isn't portable (but OTOH it isn't like you are
> >>>>>> going to be using freedreno on tegra). So why don't you go for a more
> >>>>>> targeted fix that only returns an error on hw where this is
> >>>>>> problematic?
> >>>>>
> >>>>> That's what the first versions of the patch did and Christian suggested
> >>>>> that it's not a good approach. In fact it should be not only Tegra that
> >>>>> has a broken dma-buf mapping, but apparently OMAP driver too.
> >>>>
> >>>> Hmm, I guess I'm a bit more conservative when it comes to potentially
> >>>> breaking userspace.
> >>>
> >>> If such userspace exists, then of course the mapping should continue to
> >>> work. Still will be great to know what that userpsace is.
> >>
> >> Definitely existing mesa does not have a special mmap path for
> >> imported dma-bufs (both in the case of panfrost and freedreno, I
> >> didn't check any others). The only question is whether there is a
> >> case where some app/test/etc imports a foreign dma-buf fd and then
> >> does something that would trigger mmap'ing, like readpix.
> >
> > 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?

Hmm, I would have expected it to work, at least prior to the mangling
debug thing danvet added.

Actually, I suspect with the mangling enabled, all sorts of things
would fall apart, I'm not seeing how things like sg_phys() continue to
work. But not sure if we have that enabled in anything that is
getting android CTS run on it.

BR,
-R

> 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.
>
> 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.
>
> --
> Best regards,
> Dmitry