virtio-mmio: fix guest kernel crash with SHM regions

In the kernel, virtio_gpu_init() uses virtio_get_shm_region()
since
commit 6076a9711dc5 ("drm/virtio: implement blob resources: probe for host visible region")
but vm_get_shm_region() unconditionally uses VIRTIO_MMIO_SHM_SEL to
get the address and the length of the region.

commit 38e895487afc ("virtio: Implement get_shm_region for MMIO transport"

As this is not implemented in QEMU, address and length are 0 and passed
as is to devm_request_mem_region() that triggers a crash:

  [drm:virtio_gpu_init] *ERROR* Could not reserve host visible region
  Unable to handle kernel NULL pointer dereference at virtual address (ptrval)

According to the comments in the kernel, a non existent shared region
has a length of (u64)-1.

This is what we return now with this patch to disable the region.

Signed-off-by: Laurent Vivier <[email protected]>
Message-Id: <[email protected]>
Acked-by: Gerd Hoffmann <[email protected]>
Reviewed-by: Stefano Garzarella <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

hw/virtio/virtio-mmio.c

@@ -191,6 +191,14 @@ static uint64_t virtio_mmio_read(void *opaque, hwaddr offset, unsigned size)
             return 0;
         }
         return vdev->generation;
+   case VIRTIO_MMIO_SHM_LEN_LOW:
+   case VIRTIO_MMIO_SHM_LEN_HIGH:
+        /*
+         * VIRTIO_MMIO_SHM_SEL is unimplemented
+         * according to the linux driver, if region length is -1
+         * the shared memory doesn't exist
+         */
+        return -1;
     case VIRTIO_MMIO_DEVICE_FEATURES_SEL:
     case VIRTIO_MMIO_DRIVER_FEATURES:
     case VIRTIO_MMIO_DRIVER_FEATURES_SEL: