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drm/virtio: use the DMA API for resource backing on Xen

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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: a3b815f09b ("drm/virtio: add iommu support.")
Signed-off-by: Ben Leggett <benjamin@edera.io>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20260806-virtgpu-xen-dma-v1-1-e499b345bbad@edera.io
This commit is contained in:
Benjamin Leggett 2026-08-06 18:54:22 -04:00 committed by Dmitry Osipenko
parent 61d85f99b5
commit 6a736d2f9d
3 changed files with 24 additions and 4 deletions
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20
drivers/gpu/drm/virtio/virtgpu_drv.h
View File

@ -43,6 +43,8 @@
#include <drm/drm_probe_helper.h>
#include <drm/virtgpu_drm.h>
#include <xen/xen.h>
#define DRIVER_NAME "virtio_gpu"
#define DRIVER_DESC "virtio GPU"
@ -60,6 +62,24 @@
/* See virtio_gpu_ctx_create. One additional character for NULL terminator. */
#define DEBUG_NAME_MAX_LEN 65
/*
* Whether the host must be told about resource backing pages by DMA address
* rather than guest-physical address.
*
* This mirrors vring_use_map_api() in drivers/virtio/virtio_ring.c, including
* its xen_domain() case.
*/
static inline bool virtio_gpu_use_dma_api(const struct virtio_device *vdev)
{
if (!virtio_has_dma_quirk(vdev))
return true;
if (xen_domain())
return true;
return false;
}
struct virtio_gpu_object_params {
unsigned long size;
bool dumb;

2
drivers/gpu/drm/virtio/virtgpu_object.c
View File

@ -163,7 +163,7 @@ static int virtio_gpu_object_shmem_init(struct virtio_gpu_device *vgdev,
struct virtio_gpu_mem_entry **ents,
unsigned int *nents)
{
bool use_dma_api = !virtio_has_dma_quirk(vgdev->vdev);
bool use_dma_api = virtio_gpu_use_dma_api(vgdev->vdev);
struct scatterlist *sg;
struct sg_table *pages;
int si;

6
drivers/gpu/drm/virtio/virtgpu_vq.c
View File

@ -739,7 +739,7 @@ int virtio_gpu_panic_cmd_transfer_to_host_2d(struct virtio_gpu_device *vgdev,
struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(objs->objs[0]);
struct virtio_gpu_transfer_to_host_2d *cmd_p;
struct virtio_gpu_vbuffer *vbuf;
bool use_dma_api = !virtio_has_dma_quirk(vgdev->vdev);
bool use_dma_api = virtio_gpu_use_dma_api(vgdev->vdev);
if (virtio_gpu_is_shmem(bo) && use_dma_api)
dma_sync_sgtable_for_device(vgdev->vdev->dev.parent,
@ -770,7 +770,7 @@ void virtio_gpu_cmd_transfer_to_host_2d(struct virtio_gpu_device *vgdev,
struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(objs->objs[0]);
struct virtio_gpu_transfer_to_host_2d *cmd_p;
struct virtio_gpu_vbuffer *vbuf;
bool use_dma_api = !virtio_has_dma_quirk(vgdev->vdev);
bool use_dma_api = virtio_gpu_use_dma_api(vgdev->vdev);
if (virtio_gpu_is_shmem(bo) && use_dma_api)
dma_sync_sgtable_for_device(vgdev->vdev->dev.parent,
@ -1203,7 +1203,7 @@ void virtio_gpu_cmd_transfer_to_host_3d(struct virtio_gpu_device *vgdev,
struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(objs->objs[0]);
struct virtio_gpu_transfer_host_3d *cmd_p;
struct virtio_gpu_vbuffer *vbuf;
bool use_dma_api = !virtio_has_dma_quirk(vgdev->vdev);
bool use_dma_api = virtio_gpu_use_dma_api(vgdev->vdev);
if (virtio_gpu_is_shmem(bo) && use_dma_api)
dma_sync_sgtable_for_device(vgdev->vdev->dev.parent,