Re: [PATCH v2] Revert "virtio: reject shm region if length is zero"

[Michael S. Tsirkin]

On Thu, Aug 07, 2025 at 10:03:29AM -0300, Igor Torrente wrote:
> The commit 206cc44588f7 ("virtio: reject shm region if length is zero")
> breaks the Virtio-gpu `host_visible` feature.
> 
> Right now in the Virtio-gpu code, `host_visible_region.len` is zero because
> the struct comes directly from the `kzalloc` allocation. And Virtio-gpu
> is using the `vm_get_shm_region` (drivers/virtio/virtio_mmio.c:536) to read
> the `addr` and `len` from Qemu/Crosvm.
> 
> ```
> drivers/gpu/drm/virtio/virtgpu_kms.c
> 132 vgdev = drmm_kzalloc(dev, sizeof(struct virtio_gpu_device), GFP_KERNEL);
> [...]
> 177 if (virtio_get_shm_region(vgdev->vdev, &vgdev->host_visible_region,
> 178                           VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> ```
> Now it always fails.
> 
> As the Virtio-gpu relies on the previous behavior, this patch reverts
> the offending commit.
> 
> Fixes: 206cc44588f7 ("virtio: reject shm region if length is zero` breaks the Virtio-gpu `host_visible")
> 
> This reverts commit 206cc44588f72b49ad4d7e21a7472ab2a72a83df.
> 
> Signed-off-by: Igor Torrente <igor.torrente@xxxxxxxxxxxxx>


Fixes has to be adjacent to Signed-off-by.
But anyway, I already included v1 in my tree, pls check it out
and tell me if more fixes are needed.

> ---
> v2: Improve the commit message (Michael S. Tsirkin)
> 
>  include/linux/virtio_config.h | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index b3e1d30c765b..169c7d367fac 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -329,8 +329,6 @@ static inline
>  bool virtio_get_shm_region(struct virtio_device *vdev,
>  			   struct virtio_shm_region *region, u8 id)
>  {
> -	if (!region->len)
> -		return false;
>  	if (!vdev->config->get_shm_region)
>  		return false;
>  	return vdev->config->get_shm_region(vdev, region, id);
> -- 
> 2.49.0