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

[Benjamin Leggett via B4 Relay <devnull+benjamin.edera.io@kernel.org>]

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(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
index 17a6a4d26516..09da7973387e 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.h
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
@@ -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;
diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
index 9bc0bd68c314..49899485be6f 100644
--- a/drivers/gpu/drm/virtio/virtgpu_object.c
+++ b/drivers/gpu/drm/virtio/virtgpu_object.c
@@ -173,7 +173,7 @@ 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;
diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index 905b1f42cd98..568fefd713e0 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -764,7 +764,7 @@ 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,
@@ -795,7 +795,7 @@ 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,
@@ -1231,7 +1231,7 @@ 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,

---
base-commit: bd4f284df04d76fd65e57141cb1e6e7a49e4c3cb
change-id: 20260806-virtgpu-xen-dma-559fab712250

Best regards,
-- 
Benjamin Leggett <benjamin@edera.io>