kvm tools: Support virtio indirect buffers

Indirect buffers are ring descriptors which point to more (even more)
descriptors.

This can be used to increase the effective ring capacity, which helps the
guest to batch large requests - very useful for blk devices.

This patch also enables indirect buffers for virtio-net and virtio-blk.

The patch is based on the lguest's code which does the same.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

diff --git a/tools/kvm/virtio/blk.c b/tools/kvm/virtio/blk.c
index 8c6f90b88f5c72e64011046fd4fa3764597e6c74..d1a0197d1346ad7bf0c18e6718bbad392a21350d 100644 (file)
--- a/tools/kvm/virtio/blk.c
+++ b/tools/kvm/virtio/blk.c
@@ -148,7 +148,8 @@ static u32 get_host_features(struct kvm *kvm, void *dev)
 {
        return  1UL << VIRTIO_BLK_F_SEG_MAX
                | 1UL << VIRTIO_BLK_F_FLUSH
-               | 1UL << VIRTIO_RING_F_EVENT_IDX;
+               | 1UL << VIRTIO_RING_F_EVENT_IDX
+               | 1UL << VIRTIO_RING_F_INDIRECT_DESC;
 }
 
 static void set_guest_features(struct kvm *kvm, void *dev, u32 features)

diff --git a/tools/kvm/virtio/core.c b/tools/kvm/virtio/core.c
index a6f180ed0a265532c3e9951308f5135210ff4aff..fe9d588554395aaeb2e7541259eda90df5176848 100644 (file)
--- a/tools/kvm/virtio/core.c
+++ b/tools/kvm/virtio/core.c
@@ -33,27 +33,56 @@ struct vring_used_elem *virt_queue__set_used_elem(struct virt_queue *queue, u32
        return used_elem;
 }
 
+/*
+ * 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 vring_desc *desc,
+                         unsigned int i, unsigned int max)
+{
+       unsigned int next;
+
+       /* If this descriptor says it doesn't chain, we're done. */
+       if (!(desc[i].flags & VRING_DESC_F_NEXT))
+               return max;
+
+       /* Check they're not leading us off end of descriptors. */
+       next = desc[i].next;
+       /* Make sure compiler knows to grab that: we don't want it changing! */
+       wmb();
+
+       return next;
+}
+
 u16 virt_queue__get_head_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, u16 head, struct kvm *kvm)
 {
        struct vring_desc *desc;
        u16 idx;
+       u16 max;
 
        idx = head;
        *out = *in = 0;
+       max = vq->vring.num;
+       desc = vq->vring.desc;
+
+       if (desc[idx].flags & VRING_DESC_F_INDIRECT) {
+
+               max = desc[idx].len / sizeof(struct vring_desc);
+               desc = guest_flat_to_host(kvm, desc[idx].addr);
+               idx = 0;
+       }
 
        do {
-               desc                     = virt_queue__get_desc(vq, idx);
-               iov[*out + *in].iov_base = guest_flat_to_host(kvm, desc->addr);
-               iov[*out + *in].iov_len  = desc->len;
-               if (desc->flags & VRING_DESC_F_WRITE)
+               /* Grab the first descriptor, and check it's OK. */
+               iov[*out + *in].iov_len = desc[idx].len;
+               iov[*out + *in].iov_base = guest_flat_to_host(kvm, desc[idx].addr);
+               /* If this is an input descriptor, increment that count. */
+               if (desc[idx].flags & VRING_DESC_F_WRITE)
                        (*in)++;
                else
                        (*out)++;
-               if (desc->flags & VRING_DESC_F_NEXT)
-                       idx = desc->next;
-               else
-                       break;
-       } while (1);
+       } while ((idx = next_desc(desc, idx, max)) != max);
 
        return head;
 }

diff --git a/tools/kvm/virtio/net.c b/tools/kvm/virtio/net.c
index ce34555e8d5493a384faecac8085e97a80bb44e2..3fb50541ee45575d4721f0a4196a9fcd39852ede 100644 (file)
--- a/tools/kvm/virtio/net.c
+++ b/tools/kvm/virtio/net.c
@@ -318,7 +318,8 @@ static u32 get_host_features(struct kvm *kvm, void *dev)
                | 1UL << VIRTIO_NET_F_GUEST_UFO
                | 1UL << VIRTIO_NET_F_GUEST_TSO4
                | 1UL << VIRTIO_NET_F_GUEST_TSO6
-               | 1UL << VIRTIO_RING_F_EVENT_IDX;
+               | 1UL << VIRTIO_RING_F_EVENT_IDX
+               | 1UL << VIRTIO_RING_F_INDIRECT_DESC;
 }
 
 static void set_guest_features(struct kvm *kvm, void *dev, u32 features)