Revert "virtio: reject shm region if length is zero"
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.
To fix, revert the offending commit.
Fixes: 206cc44588f7 ("virtio: reject shm region if length is zero")
Signed-off-by: Igor Torrente <igor.torrente@collabora.com>
Message-Id: <20250807124145.81816-1-igor.torrente@collabora.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 918cf25..8bf156d 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -328,8 +328,6 @@ 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);
