lguest: add support for indirect ring entries

Support the VIRTIO_RING_F_INDIRECT_DESC feature.

This is a simple matter of changing the descriptor walking
code to operate on a struct vring_desc* and supplying it
with an indirect table if detected.
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -536,20 +536,21 @@ static void *_check_pointer(unsigned long addr, unsigned int size,
 /* Each buffer in the virtqueues is actually a chain of descriptors.  This
  * function returns the next descriptor in the chain, or vq->vring.num if we're
  * at the end. */
-static unsigned next_desc(struct virtqueue *vq, unsigned int i)
+static unsigned next_desc(struct vring_desc *desc,
+			  unsigned int i, unsigned int max)
 {
 	unsigned int next;
 
 	/* If this descriptor says it doesn't chain, we're done. */
-	if (!(vq->vring.desc[i].flags & VRING_DESC_F_NEXT))
-		return vq->vring.num;
+	if (!(desc[i].flags & VRING_DESC_F_NEXT))
+		return max;
 
 	/* Check they're not leading us off end of descriptors. */
-	next = vq->vring.desc[i].next;
+	next = desc[i].next;
 	/* Make sure compiler knows to grab that: we don't want it changing! */
 	wmb();
 
-	if (next >= vq->vring.num)
+	if (next >= max)
 		errx(1, "Desc next is %u", next);
 
 	return next;
@@ -585,7 +586,8 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
 				 struct iovec iov[],
 				 unsigned int *out_num, unsigned int *in_num)
 {
-	unsigned int i, head;
+	unsigned int i, head, max;
+	struct vring_desc *desc;
 	u16 last_avail = lg_last_avail(vq);
 
 	while (last_avail == vq->vring.avail->idx) {
@@ -630,15 +632,28 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
 	/* When we start there are none of either input nor output. */
 	*out_num = *in_num = 0;
 
+	max = vq->vring.num;
+	desc = vq->vring.desc;
 	i = head;
+
+	/* If this is an indirect entry, then this buffer contains a descriptor
+	 * table which we handle as if it's any normal descriptor chain. */
+	if (desc[i].flags & VRING_DESC_F_INDIRECT) {
+		if (desc[i].len % sizeof(struct vring_desc))
+			errx(1, "Invalid size for indirect buffer table");
+
+		max = desc[i].len / sizeof(struct vring_desc);
+		desc = check_pointer(desc[i].addr, desc[i].len);
+		i = 0;
+	}
+
 	do {
 		/* Grab the first descriptor, and check it's OK. */
-		iov[*out_num + *in_num].iov_len = vq->vring.desc[i].len;
+		iov[*out_num + *in_num].iov_len = desc[i].len;
 		iov[*out_num + *in_num].iov_base
-			= check_pointer(vq->vring.desc[i].addr,
-					vq->vring.desc[i].len);
+			= check_pointer(desc[i].addr, desc[i].len);
 		/* If this is an input descriptor, increment that count. */
-		if (vq->vring.desc[i].flags & VRING_DESC_F_WRITE)
+		if (desc[i].flags & VRING_DESC_F_WRITE)
 			(*in_num)++;
 		else {
 			/* If it's an output descriptor, they're all supposed
@@ -649,9 +664,9 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
 		}
 
 		/* If we've got too many, that implies a descriptor loop. */
-		if (*out_num + *in_num > vq->vring.num)
+		if (*out_num + *in_num > max)
 			errx(1, "Looped descriptor");
-	} while ((i = next_desc(vq, i)) != vq->vring.num);
+	} while ((i = next_desc(desc, i, max)) != max);
 
 	return head;
 }
@@ -1331,6 +1346,8 @@ static void setup_tun_net(char *arg)
 	add_feature(dev, VIRTIO_NET_F_HOST_TSO4);
 	add_feature(dev, VIRTIO_NET_F_HOST_TSO6);
 	add_feature(dev, VIRTIO_NET_F_HOST_ECN);
+	/* We handle indirect ring entries */
+	add_feature(dev, VIRTIO_RING_F_INDIRECT_DESC);
 	set_config(dev, sizeof(conf), &conf);
 
 	/* We don't need the socket any more; setup is done. */