[Bug] virtio-gpu broken with qemu/kvm on arm64 on kernel 5.5+

[Guillaume Gardet]

Hi,

With (guest) kernel 5.5+ with qemu/kvm on arm64, I get lots of display corruptions leading to this kind of screen:
https://openqa.opensuse.org/tests/1174521#step/yast2_i/24

I git bisected it to commit c66df701e783bc666593e6e665f13670760883ee
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drm/virtio: switch from ttm to gem shmem helpers

  virtio-gpu basically needs a sg_table for the bo, to tell the host where
  the backing pages for the object are.  So the gem shmem helpers are a
  perfect fit.  Some drm_gem_object_funcs need thin wrappers to update the
  host state, but otherwise the helpers handle everything just fine.
  
  Once the fencing was sorted the switch was surprisingly easy and for the
  most part just removing the ttm code.
  
  v4: fix drm_gem_object_funcs name.
  
  Signed-off-by: Gerd Hoffmann <kraxel at redhat.com>
  Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
  Reviewed-by: Chia-I Wu <olvaffe at gmail.com>
  Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-15-kraxel@redhat.com
**********

I also tested kernel 5.6-rc2 which has the same bug.
Without kvm, the display is fine.

Regards,
Guillaume