Re: [PATCH v3 11/12] drm/virtio: implement context init: add virtio_gpu_fence_event

[Gurchetan Singh <gurchetansingh@chromium.org>]

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On Fri, Nov 19, 2021 at 9:38 AM Rob Clark <robdclark@gmail.com> wrote:

> On Thu, Nov 18, 2021 at 12:53 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Tue, Nov 16, 2021 at 06:31:10PM -0800, Gurchetan Singh wrote:
> > > On Tue, Nov 16, 2021 at 7:43 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> > >
> > > > On Mon, Nov 15, 2021 at 07:26:14PM +0000, Kasireddy, Vivek wrote:
> > > > > Hi Daniel, Greg,
> > > > >
> > > > > If it is the same or a similar crash reported here:
> > > > >
> > > >
> https://lists.freedesktop.org/archives/dri-devel/2021-November/330018.html
> > > > > and here:
> > > >
> https://lists.freedesktop.org/archives/dri-devel/2021-November/330212.html
> > > > > then the fix is already merged:
> > > > >
> > > >
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d89c0c8322ecdc9a2ec84b959b6f766be082da76
> > >
> > > Yeah but that still leaves the problem of why exaxtly virtgpu is
> > > > reinventing drm_poll here?
> > >
> > >
> > > > Can you please replace it with drm_poll like all other drivers,
> including
> > > > the ones that have private events?
> > > >
> > >
> > > Hi Daniel,
> > >
> > > Allow me to explain the use case a bit.  It's for when virtgpu KMS is
> not
> > > used, but a special Wayland compositor does wayland passthrough
> instead:
> > >
> > >
> https://www.youtube.com/watch?v=WwrXqDERFm8https://www.youtube.com/watch?v=EkNBsBx501Q
> > >
> > > This technique has gained much popularity in the virtualized laptop
> > > space, where it offers better performance/user experience than virtgpu
> > > KMS.  The relevant paravirtualized userspace is "Sommelier":
> > >
> > >
> https://chromium.googlesource.com/chromiumos/platform2/+/main/vm_tools/sommelier/
> > >
> https://chromium.googlesource.com/chromiumos/platform2/+/main/vm_tools/sommelier/virtualization/virtgpu_channel.cc
> > >
> > > Previously, we were using the out-of-tree virtio-wl device and there
> > > were many discussions on how we could get this upstream:
> > >
> > >
> https://lists.freedesktop.org/archives/dri-devel/2017-December/160309.html
> > > https://lists.oasis-open.org/archives/virtio-dev/202002/msg00005.html
> > >
> > > Extending virtgpu was deemed the least intrusive option:
> > >
> > > https://www.spinics.net/lists/kvm/msg159206.html
> > >
> > > We ultimately settled on the context type abstraction and used
> > > virtio_gpu_poll to tell the guest "hey, we have a Wayland event".  The
> > > host response is actually in a buffer of type BLOB_MEM_GUEST.
> > >
> > > It is possible to use drm_poll(..), but that would have to be
> > > accompanied by a drm_read(..).  You'll need to define a dummy
> > > VIRTGPU_EVENT_FENCE_SIGNALED in the uapi too.
> > >
> > > That's originally how I did it, but some pointed out that's
> > > unnecessary since the host response is in the BLOB_MEM_GUEST buffer
> > > and virtgpu event is a dummy event.  So we decided just to modify
> > > virtio_gpu_poll(..) to have the desired semantics in that case.
> > >
> > > For the regular virtio-gpu KMS path, things remain unchanged.
> > >
> > > There are of course other ways to do it (perhaps polling a dma_fence),
> > > but that was the cleanest way we could find.
> > >
> > > It's not rare for virtio to "special things" (see virtio_dma_buf_ops,
> > > virtio_dma_ops), since they are in fake devices.
> >
> > These are all internal interfaces, not uapi.
> >
> > > We're open to other ideas, but hopefully that answers some of your
> > > questions.
> >
> > Well for one, why does the commit message not explain any of this. You're
> > building uapi, which is forever, it's paramount all considerations are
> > properly explained.
> >
> > Second, I really don't like that youre redefining poll semantics in
> > incompatible ways from all other drm drivers. If you want special poll
> > semantics then just create a sperate fd for that (or a dma_fence or
> > whatever, maybe that saves some typing), but bending the drm fd semantics
> > is no good at all. We have tons of different fd with their dedicated
> > semantics in drm, trying to shoehorn it all into one just isn't very good
> > design.
> >
> > Or do the dummy event which is just the event code, but does not contain
> > any data. Either is fine with me.
> >
> > Can you pls do this asap? I really don't want to bake this in as uapi
> > which we then have to quirk and support forever. I'd say revert for -rc2
> > of these two and then maybe sort it out properly in -next.
>
> I think as a general rule, if there is not consensus about uabi
> change, even if it is just a semantic change, then revert and reland
> later is ok..
>
> As far as dummy VIRTGPU_EVENT_FENCE_SIGNALED.. that doesn't entirely
> sound like a bad thing to me.  Actually, it sounds like a good thing..
> it makes it more explicit what is going on.  And would avoid confusing
> a different userspace polling for kms related events expecting to be
> able to do a read.
>

If dummy events work, then it's actually not a big change to make.  Expect
patches in the upcoming business days.


>
> BR,
> -R
>
> > Cheers, Daniel
> > >
> > >
> > > > Thanks, Daniel
> > > >
> > > > >
> > > > > Thanks,
> > > > > Vivek
> > > > >
> > > > > > On Sat, Nov 13, 2021 at 03:51:48PM +0100, Greg KH wrote:
> > > > > > > On Tue, Sep 21, 2021 at 04:20:23PM -0700, Gurchetan Singh
> wrote:
> > > > > > > > Similar to DRM_VMW_EVENT_FENCE_SIGNALED.  Sends a pollable
> event
> > > > > > > > to the DRM file descriptor when a fence on a specific ring is
> > > > > > > > signaled.
> > > > > > > >
> > > > > > > > One difference is the event is not exposed via the UAPI --
> this is
> > > > > > > > because host responses are on a shared memory buffer of type
> > > > > > > > BLOB_MEM_GUEST [this is the common way to receive responses
> with
> > > > > > > > virtgpu].  As such, there is no context specific read(..)
> > > > > > > > implementation either -- just a poll(..) implementation.
> > > > > > > >
> > > > > > > > Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
> > > > > > > > Acked-by: Nicholas Verne <nverne@chromium.org>
> > > > > > > > ---
> > > > > > > >  drivers/gpu/drm/virtio/virtgpu_drv.c   | 43
> > > > +++++++++++++++++++++++++-
> > > > > > > >  drivers/gpu/drm/virtio/virtgpu_drv.h   |  7 +++++
> > > > > > > >  drivers/gpu/drm/virtio/virtgpu_fence.c | 10 ++++++
> > > > > > > >  drivers/gpu/drm/virtio/virtgpu_ioctl.c | 34
> ++++++++++++++++++++
> > > > > > > >  4 files changed, 93 insertions(+), 1 deletion(-)
> > > > > > >
> > > > > > > This commit seems to cause a crash in a virtual drm gpu driver
> for
> > > > > > > Android.  I have reverted this, and the next commit in the
> series
> > > > from
> > > > > > > Linus's tree and all is good again.
> > > > > > >
> > > > > > > Any ideas?
> > > > > >
> > > > > > Well no, but also this patch looks very questionable of
> hand-rolling
> > > > > > drm_poll. Yes you can do driver private events like
> > > > > > DRM_VMW_EVENT_FENCE_SIGNALED, that's fine. But you really should
> not
> > > > need
> > > > > > to hand-roll the poll callback. vmwgfx (which generally is a
> very old
> > > > > > driver which has lots of custom stuff, so not a great example)
> doesn't
> > > > do
> > > > > > that either.
> > > > > >
> > > > > > So that part should go no matter what I think.
> > > > > > -Daniel
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch
> > > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
>

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