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

[Daniel Vetter]

On Thu, Jan 05, 2023 at 11:10:23PM +0900, Yi Xie wrote:
> 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.

Ah ok. For next time around, copy a link to the comment you want, e.g.

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5830#note_582477

The 3 dots menu on each comments has an option to copy that link tag. That
also highlights the right comment.

On this issue, I'm concurring with Emil:

"- the import is broken
"IMHO that should be fixed, regardless of the rest"

The same should be done here. Unless it's a very special device, we should
be able to import vgem buffers.
-Daniel

> 
> > -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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch