vhost: avoid to start/stop virtqueue which is not ready

In our Armv8a server, we try to configure the vhost scsi but fail
to boot up the guest (-machine virt-2.10). The guest's boot failure
is very early, even earlier than grub.

There are 3 virtqueues (ctrl, event and cmd) for virtio scsi device,
but ovmf and seabios will only set the physical address for the 3rd
one (cmd). Then in vhost_virtqueue_start(), virtio_queue_get_desc_addr
will be 0 for ctrl and event vq when qemu negotiates with ovmf. So
vhost_memory_map fails with ENOMEM.

This patch just fixs it by early quitting the virtqueue start/stop
when virtio_queue_get_desc_addr is 0.

Btw, after guest kernel starts, all the 3 queues will be initialized
and set address correctly.

Already tested on Arm64 and X86_64 qemu.
Signed-off-by: NJia He <jia.he@hxt-semitech.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost.c

@@ -345,6 +345,10 @@ static int vhost_verify_ring_mappings(struct vhost_dev *dev,
     for (i = 0; i < dev->nvqs; ++i) {
         struct vhost_virtqueue *vq = dev->vqs + i;
 
+        if (vq->desc_phys == 0) {
+            continue;
+        }
+
         j = 0;
         r = vhost_verify_ring_part_mapping(
                 vq->desc, vq->desc_phys, vq->desc_size,
@@ -881,6 +885,11 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
     };
     struct VirtQueue *vvq = virtio_get_queue(vdev, idx);
 
+    a = virtio_queue_get_desc_addr(vdev, idx);
+    if (a == 0) {
+        /* Queue might not be ready for start */
+        return 0;
+    }
 
     vq->num = state.num = virtio_queue_get_num(vdev, idx);
     r = dev->vhost_ops->vhost_set_vring_num(dev, &state);
@@ -906,7 +915,7 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
     }
 
     vq->desc_size = s = l = virtio_queue_get_desc_size(vdev, idx);
-    vq->desc_phys = a = virtio_queue_get_desc_addr(vdev, idx);
+    vq->desc_phys = a;
     vq->desc = vhost_memory_map(dev, a, &l, 0);
     if (!vq->desc || l != s) {
         r = -ENOMEM;
@@ -989,6 +998,13 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
         .index = vhost_vq_index,
     };
     int r;
+    int a;
+
+    a = virtio_queue_get_desc_addr(vdev, idx);
+    if (a == 0) {
+        /* Don't stop the virtqueue which might have not been started */
+        return;
+    }
 
     r = dev->vhost_ops->vhost_get_vring_base(dev, &state);
     if (r < 0) {