Re: `virtio: reject shm region if length is zero` breaks virtio-gpu host_visible feature

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

On Wed, Aug 06, 2025 at 03:34:55PM -0300, Igor Torrente wrote:
> Hello,
> 
> The commit `206cc44588f7 virtio: reject shm region if length is zero` breaks the Virtio-gpu `host_visible` feature.
> 
> As you can see in the snippet below, host_visible_region is zero because of the `kzalloc`. 
> It's 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.
> 
> I don't know exactly what issue this patch is trying to solve, but right now Virtio-gpu relies upon the previous behavior. 
> Can we just revert it?
> 
> BR,
> ----
> Igor Torrente


Pls go ahead and post the revert. We'll see what happens.

-- 
MST