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

[Benjamin Leggett <benjamin@edera.io>]

Dmitry Osipenko <dmitry.osipenko@collabora.com> writes:

> On 8/7/26 01:54, Benjamin Leggett via B4 Relay wrote:
>> From: Benjamin Leggett <benjamin@edera.io>
>> 
>> 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@edera.io>
>> ---
>>  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!

Thanks, a proposed followup for the Sashiko-bot reported issue
submitted: https://lore.kernel.org/dri-devel/20260814-virtgpu-from-host-sync-v4-1-64dd736b1779@edera.io/