linux-kernel - Re: [PATCH v6 15/18] drm/virtio: rework virtio_gpu_transfer_to_host

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Date:   Fri, 5 Jul 2019 11:05:53 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Chia-I Wu <olvaffe@...il.com>
Cc:     ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 15/18] drm/virtio: rework
 virtio_gpu_transfer_to_host_ioctl fencing

On Thu, Jul 04, 2019 at 12:08:14PM -0700, Chia-I Wu wrote:
> On Thu, Jul 4, 2019 at 4:51 AM Gerd Hoffmann <kraxel@...hat.com> wrote:
> >
> >   Hi,
> >
> > > >         convert_to_hw_box(&box, &args->box);
> > > >         if (!vgdev->has_virgl_3d) {
> > > >                 virtio_gpu_cmd_transfer_to_host_2d
> > > > -                       (vgdev, qobj, offset,
> > > > +                       (vgdev, gem_to_virtio_gpu_obj(objs->objs[0]), offset,
> > > >                          box.w, box.h, box.x, box.y, NULL);
> > > > +               virtio_gpu_array_put_free(objs);
> > > Don't we need this in non-3D case as well?
> >
> > No, ...
> >
> > > >                 virtio_gpu_cmd_transfer_to_host_3d
> > > > -                       (vgdev, qobj,
> > > > +                       (vgdev,
> > > >                          vfpriv ? vfpriv->ctx_id : 0, offset,
> > > > -                        args->level, &box, fence);
> > > > -               reservation_object_add_excl_fence(qobj->base.base.resv,
> > > > -                                                 &fence->f);
> > > > +                        args->level, &box, objs, fence);
> >
> > ... 3d case passes the objs list to virtio_gpu_cmd_transfer_to_host_3d,
> > so it gets added to the vbuf and released when the command is finished.
> Why doesn't this apply to virtio_gpu_cmd_transfer_to_host_2d?

Hmm, yes, makes sense to handle both the same way.

With virgl=off qemu processes the commands from the guest
synchronously, so it'll work fine as-is.  So you can't hit
the theoretical race window in practice.  But depending
on that host-side implementation detail isn't a good idea
indeed.

cheers,
  Gerd