Re: [PATCH] drm/virtio: fix host visible memory detection in virtio-gpu

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 27, 2025 at 04:51:37PM +0300, Dmitry Osipenko wrote:
> On 8/27/25 16:33, Michael S. Tsirkin wrote:
> > On Wed, Aug 27, 2025 at 03:52:05PM +0300, Dmitry Osipenko wrote:
> >> On 8/27/25 11:12, Honglei Huang wrote:
> >>> From: Honglei Huang <Honglei1.Huang@amd.com>
> >>>
> >>> Commit 206cc44588f7 ("virtio: reject shm region if length is zero")
> >>> enhanced the validation in virtio_get_shm_region() by adding a check
> >>> for a zero-length shared memory region.
> >>>
> >>> It is performed before the underlying transport's .get_shm_region()
> >>> implementation is called. This creates an issue in the virtio-gpu
> >>> driver, where the `region` struct is part of a larger structure
> >>> that is zero-initialized by drmm_kzalloc().
> >>>
> >>> Consequently, the `len` field is 0 at the time of the check, causing
> >>> virtio_get_shm_region() to return false prematurely. This prevents the
> >>> host visible memory feature from being enabled, even when the device
> >>> supports it.
> >>>
> >>> To resolve this, this patch bypasses the inline helper and calls the
> >>> underlying vdev->config->get_shm_region() function pointer directly.
> >>> This ensures that the region's parameters are checked only after they
> >>> have been populated by the transport, aligning with the intended logic.
> >>>
> >>> Signed-off-by: Honglei Huang <Honglei1.Huang@amd.com>
> >>> ---
> >>>  drivers/gpu/drm/virtio/virtgpu_kms.c | 6 ++++--
> >>>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> >>> index 7dfb2006c561..ed5981248302 100644
> >>> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> >>> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> >>> @@ -174,8 +174,10 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
> >>>  	if (virtio_has_feature(vgdev->vdev, VIRTIO_GPU_F_RESOURCE_BLOB)) {
> >>>  		vgdev->has_resource_blob = true;
> >>>  	}
> >>> -	if (virtio_get_shm_region(vgdev->vdev, &vgdev->host_visible_region,
> >>> -				  VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> >>> +	if (vgdev->vdev->config->get_shm_region &&
> >>> +	    vgdev->vdev->config->get_shm_region(
> >>> +		    vgdev->vdev, &vgdev->host_visible_region,
> >>> +		    VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> >>>  		if (!devm_request_mem_region(&vgdev->vdev->dev,
> >>>  					     vgdev->host_visible_region.addr,
> >>>  					     vgdev->host_visible_region.len,
> >>
> >> Hi, virtio_get_shm_region() change has been reverted by [1]. Don't think
> >> anything else needs to be done.
> >>
> >> [1]
> >> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20250827&id=ced17ee32a9988b8a260628e7c31a100d7dc082e
> >>
> >> +cc Michael Tsirkin
> >>
> >> Might be only good to send a stable kernel PR with that revert. I see
> >> patch available only in linux-next, while stable kernels need to be
> >> fixed sooner.
> > 
> > sooner than what?
> 
> Next 6.17 kernel release. I see patch in the linux-next branch. Often
> there is a -fixes branch for patches that go into RC kernel, but I don't
> see one in your vhost kernel tree. Will the revert land into 6.17-rc4?
> Everything is good if yes.

Should go into rc4 or rc5, yes.


> -- 
> Best regards,
> Dmitry