Re: [PATCH] drm/virtio: use the DMA API for resource backing on Xen

[Dmitry Osipenko]

On 8/7/26 01:54, Benjamin Leggett via B4 Relay wrote:
> From: Benjamin Leggett <benjamin@xxxxxxxx>
> 
> On a Xen PV domain page addresses bear no relation to the real machine
> addresses the host would have to use to reach it.
> virtio_ring.c handles this correctly, vring_use_map_api() returns true
> for any xen_domain() regardless of VIRTIO_F_ACCESS_PLATFORM.
> 
> virtio-gpu makes the same decision independently, but its copy
> looks only at the feature bit:
> 
> 	bool use_dma_api = !virtio_has_dma_quirk(vgdev->vdev);
> 
> QEMU does not set iommu_platform on virtio-vga by default, so
> VIRTIO_F_ACCESS_PLATFORM is not negotiated, use_dma_api is false, and
> virtio_gpu_object_shmem_init() describes the framebuffer's backing pages
> to the host with sg_phys().  Those are guest-physical addresses. In a PV
> domain they resolve, on the host side, to pages belonging to some other
> domain, so the host scans out unrelated memory.
> 
> Move the decision into virtio_gpu_use_dma_api() and give it the
> xen_domain() check, like vring_use_map_api() has. This
> additionally enables the dma_sync_sgtable_for_device() calls in
> virtgpu_vq.c, which are required for correctness whenever swiotlb
> is in play.
> 
> Reproduced with a Xen 4.21 PV dom0 nested inside QEMU 8.2 with
> virtio-vga, on both a distro 6.8 kernel and 6.18 LTS. A PVH dom0
> works fine and doesn't need this fix because it is identity-mapped,
> only PV dom0s are affected.
> 
> Fixes: a3b815f09bb8 ("drm/virtio: add iommu support.")
> Signed-off-by: Ben Leggett <benjamin@xxxxxxxx>
> ---
>  drivers/gpu/drm/virtio/virtgpu_drv.h    | 20 ++++++++++++++++++++
>  drivers/gpu/drm/virtio/virtgpu_object.c |  2 +-
>  drivers/gpu/drm/virtio/virtgpu_vq.c     |  6 +++---
>  3 files changed, 24 insertions(+), 4 deletions(-)

Feel free to send follow-up patches to fix problems reported by the
Sashiko bot.

Applied to drm-misc-fixes, thanks!

-- 
Best regards,
Dmitry