Linux Virtualization - Jul 2019 - [PATCH v6 06/18] drm/virtio: remove ttm calls from in virtio_gpu_object_{reserve,unreserve}

Hi,
> > -       r = ttm_bo_reserve(&bo->tbo, true, false, NULL);
> > +       r =
reservation_object_lock_interruptible(bo->gem_base.resv, NULL);
> Can you elaborate a bit about how TTM keeps the BOs alive in, for
> example, virtio_gpu_transfer_from_host_ioctl?  In that function, only
> three TTM functions are called: ttm_bo_reserve, ttm_bo_validate, and
> ttm_bo_unreserve.  I am curious how they keep the BO alive.
It can't go away between reserve and unreserve, and I think it also
can't be evicted then.  Havn't checked how ttm implements that.

cheers,
  Gerd

On Thu, Jul 4, 2019 at 4:10 AM Gerd Hoffmann <kraxel at redhat.com>
wrote:
>
>   Hi,
>
> > > -       r = ttm_bo_reserve(&bo->tbo, true, false, NULL);
> > > +       r =
reservation_object_lock_interruptible(bo->gem_base.resv, NULL);
> > Can you elaborate a bit about how TTM keeps the BOs alive in, for
> > example, virtio_gpu_transfer_from_host_ioctl?  In that function, only
> > three TTM functions are called: ttm_bo_reserve, ttm_bo_validate, and
> > ttm_bo_unreserve.  I am curious how they keep the BO alive.
>
> It can't go away between reserve and unreserve, and I think it also
> can't be evicted then.  Havn't checked how ttm implements that.
Hm, but the vbuf using the BO outlives the reserve/unreserve section.
The NO_EVICT flag applies only when the BO is still alive.  Someone
needs to hold a reference to the BO to keep it alive, otherwise the BO
can go away before the vbuf is retired.

I can be wrong, but on the other hand, it seems fine for a BO to go
away before the vbuf using it is retired.  When that happens, the
driver emits a RESOURCE_UNREF vbuf which is *after* the original vbuf.

>
> cheers,
>   Gerd
>

On Thu, Jul 04, 2019 at 12:17:48PM -0700, Chia-I Wu
wrote:
> On Thu, Jul 4, 2019 at 4:10 AM Gerd Hoffmann <kraxel at redhat.com>
wrote:
> >
> >   Hi,
> >
> > > > -       r = ttm_bo_reserve(&bo->tbo, true, false,
NULL);
> > > > +       r =
reservation_object_lock_interruptible(bo->gem_base.resv, NULL);
> > > Can you elaborate a bit about how TTM keeps the BOs alive in, for
> > > example, virtio_gpu_transfer_from_host_ioctl?  In that function,
only
> > > three TTM functions are called: ttm_bo_reserve, ttm_bo_validate,
and
> > > ttm_bo_unreserve.  I am curious how they keep the BO alive.
> >
> > It can't go away between reserve and unreserve, and I think it
also
> > can't be evicted then.  Havn't checked how ttm implements
that.
> Hm, but the vbuf using the BO outlives the reserve/unreserve section.
> The NO_EVICT flag applies only when the BO is still alive.  Someone
> needs to hold a reference to the BO to keep it alive, otherwise the BO
> can go away before the vbuf is retired.
Note that patches 14+15 rework virtio_gpu_transfer_*_ioctl to keep
gem reference until the command is finished and patch 17 drops
virtio_gpu_object_{reserve,unreserve} altogether.

Maybe I should try to reorder the series, then squash 6+17 to reduce
confusion.  I suspect that'll cause quite a few conflicts though ...

cheers,
  Gerd