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

[Igor Torrente <igor.torrente@collabora.com>]

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@collabora.com>
---
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