[PATCH] drm/vkms: Add a DRM render node to vkms

[Yi Xie]

On Thu, Jan 5, 2023 at 10:48 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Thu, Jan 05, 2023 at 09:52:26PM +0900, Yi Xie wrote:
> > > This doesn't sound like a good idea to me. Devices without render
> > > capabilities should not fake it.
> > >
> > > User-space (e.g. wlroots) relies on "no render node" to enable
> > > software rendering (Pixman instead of GL).
> >
> > We have virtgpu driver that exports a render node even when virgl is
> > not supported.
> > Mesa has special code path to enable software rendering on it:
> > https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/egl/drivers/dri2/platform_device.c#L296
> > We can do the same for vkms to force software rendering.
>
> Yeah that is the old kmsro mesa issue, for every combination of kms and
> gem device you need one to make this work.
>
> > On Thu, Jan 5, 2023 at 8:36 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > >
> > > On Thu, Jan 05, 2023 at 02:23:25PM +0900, Yi Xie wrote:
> > > > Some libraries including Mesa and virglrenderer require a render node to
> > > > fully function. By adding a render node to vkms those libraries will
> > > > work properly, supporting use cases like running crosvm with virgl GPU
> > > > support via llvmpipe on a headless virtual machine.
> > >
> > > This is what vgem exists for. More or less at least ... I'm honestly not
> > > really understanding what you're trying to fix here, it sounds a bit like
> > > userspace being stupid.
> > > -Daniel
> > The problem with vgem is that it crashes llvmpipe while working with vkms.
> > Looks like it's due to the same reason as described in this thread in Mesa:
> > https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5830
>
> I'm not finding any bug description in there and how/why something
> crashes?

The discussion is in the comment section under the first comment by
Emil Velikov.
It's folded by default (inside "6 replies" at the bottom).

>
> > Importing buffers allocated by vgem to vkms seems to be unexpected and
> > causes the crash. If we create a render node on vkms then llvmpipe will use
> > vkms to allocate buffers and it no longer crashes.
>
> Uh importing vgem into virtio might not work because those sometimes need
> special buffers iirc. But importing vgem into vkms really should work,
> there's no technical reason it cannot. If it doesn't, then the right fix
> would be to fix that, not paper around it.

The crash stack trace looks like this:
https://gist.github.com/imxieyi/03053ae79cee2e614850fd41829e1da2

Even if we fix the crash issue with vgem, we still need to workaround
quite a few
places that has explicitly blocked vgem. A notable example is virglrenderer:
https://gitlab.freedesktop.org/virgl/virglrenderer/-/blob/master/src/vrend_winsys_gbm.c#L121

Actually I have tried to force running virglrenderer on vgem and it
didn't work. I
didn't look into why it wasn't working but I guess that's the reason
for blocking
vgem in the first place. Virglrenderer works well on vkms with render node
enabled though.

> -Daniel
>
> >
> > > >
> > > > Signed-off-by: Yi Xie <yixie at google.com>
> > > > ---
> > > >  drivers/gpu/drm/vkms/vkms_drv.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > > > index 293dbca50c31..8eea5d4dece8 100644
> > > > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > > > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > > > @@ -113,7 +113,7 @@ static void vkms_config_debugfs_init(struct drm_minor *minor)
> > > >  }
> > > >
> > > >  static const struct drm_driver vkms_driver = {
> > > > -     .driver_features        = DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_GEM,
> > > > +     .driver_features        = DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_GEM | DRIVER_RENDER,
> > > >       .release                = vkms_release,
> > > >       .fops                   = &vkms_driver_fops,
> > > >       DRM_GEM_SHMEM_DRIVER_OPS,
> > > > --
> > > > 2.39.0.314.g84b9a713c41-goog
> > > >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch