virtio: support reserved vqs

virtio network device multiqueue support reserves
vq 3 for future use (useful both for future extensions and to make it
pretty - this way receive vqs have even and transmit - odd numbers).
Make it possible to skip initialization for
specific vq numbers by specifying NULL for name.
Document this usage as well as (existing) NULL callback.

Drivers using this not coded up yet, so I simply tested
with virtio-pci and verified that this patch does
not break existing drivers.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/lguest_device.c

@@ -263,6 +263,9 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	struct virtqueue *vq;
 	int err;
 
+	if (!name)
+		return NULL;
+
 	/* We must have this many virtqueues. */
 	if (index >= ldev->desc->num_vq)
 		return ERR_PTR(-ENOENT);

drivers/remoteproc/remoteproc_virtio.c

@@ -84,6 +84,9 @@ static struct virtqueue *rp_find_vq(struct virtio_device *vdev,
 	if (id >= ARRAY_SIZE(rvdev->vring))
 		return ERR_PTR(-EINVAL);
 
+	if (!name)
+		return NULL;
+
 	ret = rproc_alloc_vring(rvdev, id);
 	if (ret)
 		return ERR_PTR(ret);

drivers/s390/kvm/kvm_virtio.c

@@ -190,6 +190,9 @@ static struct virtqueue *kvm_find_vq(struct virtio_device *vdev,
 	if (index >= kdev->desc->num_vq)
 		return ERR_PTR(-ENOENT);
 
+	if (!name)
+		return NULL;
+
 	config = kvm_vq_config(kdev->desc)+index;
 
 	err = vmem_add_mapping(config->address,

drivers/virtio/virtio_mmio.c

@@ -306,6 +306,9 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned index,
 	unsigned long flags, size;
 	int err;
 
+	if (!name)
+		return NULL;
+
 	/* Select the queue we're interested in */
 	writel(index, vm_dev->base + VIRTIO_MMIO_QUEUE_SEL);
 

drivers/virtio/virtio_pci.c

@@ -555,7 +555,10 @@ static int vp_try_to_find_vqs(struct virtio_device *vdev, unsigned nvqs,
 	vp_dev->per_vq_vectors = per_vq_vectors;
 	allocated_vectors = vp_dev->msix_used_vectors;
 	for (i = 0; i < nvqs; ++i) {
-		if (!callbacks[i] || !vp_dev->msix_enabled)
+		if (!names[i]) {
+			vqs[i] = NULL;
+			continue;
+		} else if (!callbacks[i] || !vp_dev->msix_enabled)
 			msix_vec = VIRTIO_MSI_NO_VECTOR;
 		else if (vp_dev->per_vq_vectors)
 			msix_vec = allocated_vectors++;

include/linux/virtio_config.h

@@ -84,7 +84,9 @@
  *	nvqs: the number of virtqueues to find
  *	vqs: on success, includes new virtqueues
  *	callbacks: array of callbacks, for each virtqueue
+ *		include a NULL entry for vqs that do not need a callback
  *	names: array of virtqueue names (mainly for debugging)
+ *		include a NULL entry for vqs unused by driver
  *	Returns 0 on success or error status
  * @del_vqs: free virtqueues found by find_vqs().
  * @get_features: get the array of feature bits for this device.