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From: Gurchetan Singh <gurchetansingh@chromium.org>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Greg KH <gregkh@linuxfoundation.org>,
	"kraxel@redhat.com" <kraxel@redhat.com>,
	"Kasireddy, Vivek" <vivek.kasireddy@intel.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"virtio-dev@lists.oasis-open.org"
	<virtio-dev@lists.oasis-open.org>
Subject: Re: [PATCH v3 11/12] drm/virtio: implement context init: add virtio_gpu_fence_event
Date: Tue, 16 Nov 2021 18:31:10 -0800	[thread overview]
Message-ID: <CAAfnVBms1Bi8MnaCZVv=4dgoG+REVzZ-zFq-hRQ-4tCzYBrDdA@mail.gmail.com> (raw)
In-Reply-To: <YZPRhZDcU3/VYWgb@phenom.ffwll.local>

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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.

We're open to other ideas, but hopefully that answers some of your
questions.


> 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
>

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  reply	other threads:[~2021-11-17  2:31 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-21 23:20 [PATCH v3 00/12] Context types, v3 Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 01/12] virtio-gpu api: multiple context types with explicit initialization Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 02/12] drm/virtgpu api: create context init feature Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 03/12] drm/virtio: implement context init: track valid capabilities in a mask Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 04/12] drm/virtio: implement context init: probe for feature Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 05/12] drm/virtio: implement context init: support init ioctl Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 06/12] drm/virtio: implement context init: track {ring_idx, emit_fence_info} in virtio_gpu_fence Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 07/12] drm/virtio: implement context init: plumb {base_fence_ctx, ring_idx} to virtio_gpu_fence_alloc Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 08/12] drm/virtio: implement context init: stop using drv->context when creating fence Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 09/12] drm/virtio: implement context init: allocate an array of fence contexts Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 10/12] drm/virtio: implement context init: handle VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 11/12] drm/virtio: implement context init: add virtio_gpu_fence_event Gurchetan Singh
2021-11-13 14:51   ` Greg KH
2021-11-15 14:16     ` Daniel Vetter
2021-11-15 19:26       ` Kasireddy, Vivek
2021-11-16 15:43         ` Daniel Vetter
2021-11-17  2:31           ` Gurchetan Singh [this message]
2021-11-18  8:53             ` Daniel Vetter
2021-11-19 17:43               ` Rob Clark
2021-11-20  1:38                 ` Gurchetan Singh
2021-09-21 23:20 ` [PATCH v3 12/12] drm/virtio: implement context init: advertise feature to userspace Gurchetan Singh
2021-09-29  7:33 ` [PATCH v3 00/12] Context types, v3 Gerd Hoffmann

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