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Tue, 18 Aug 2026 10:48:40 -0700 (PDT) From: Benjamin Leggett To: Dmitry Osipenko Cc: David Airlie , Gerd Hoffmann , Gurchetan Singh , Chia-I Wu , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Simona Vetter , dri-devel@lists.freedesktop.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] drm/virtio: use the DMA API for resource backing on Xen In-Reply-To: <657a88d7-e1d4-46ee-9b79-b33144e749dd@collabora.com> References: <20260806-virtgpu-xen-dma-v1-1-e499b345bbad@edera.io> <657a88d7-e1d4-46ee-9b79-b33144e749dd@collabora.com> User-Agent: mu4e 1.12.12; emacs 32.0.50 Date: Tue, 18 Aug 2026 13:48:37 -0400 Message-ID: <875x17tj4a.fsf@edera.io> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Dmitry Osipenko writes: > On 8/7/26 01:54, Benjamin Leggett via B4 Relay wrote: >> From: Benjamin Leggett >> >> 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 >> --- >> 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/