virtio_ring: support event idx feature

Support for the new event idx feature:
1. When enabling interrupts, publish the current avail index
   value to the host to get interrupts on the next update.
2. Use the new avail_event feature to reduce the number
   of exits from the guest.

Simple test with the simulator:

[virtio]# time ./virtio_test
spurious wakeus: 0x7

real    0m0.169s
user    0m0.140s
sys     0m0.019s
[virtio]# time ./virtio_test --no-event-idx
spurious wakeus: 0x11

real    0m0.649s
user    0m0.295s
sys     0m0.335s

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

1 files changed, 24 insertions, 2 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index b0043fb26a4d..a5fadc40c448 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -82,6 +82,9 @@ struct vring_virtqueue
         /* Host supports indirect buffers */
         bool indirect;
 
+        /* Host publishes avail event idx */
+        bool event;
+
         /* Number of free buffers */
         unsigned int num_free;
         /* Head of free buffer list. */
@@ -237,18 +240,22 @@ EXPORT_SYMBOL_GPL(virtqueue_add_buf_gfp);
 void virtqueue_kick(struct virtqueue *_vq)
 {
         struct vring_virtqueue *vq = to_vvq(_vq);
+        u16 new, old;
         START_USE(vq);
         /* Descriptors and available array need to be set before we expose the
          * new available array entries. */
         virtio_wmb();
 
-        vq->vring.avail->idx += vq->num_added;
+        old = vq->vring.avail->idx;
+        new = vq->vring.avail->idx = old + vq->num_added;
         vq->num_added = 0;
 
         /* Need to update avail index before checking if we should notify */
         virtio_mb();
 
-        if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
+        if (vq->event ?
+            vring_need_event(vring_avail_event(&vq->vring), new, old) :
+            !(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
                 /* Prod other side to tell it about changes. */
                 vq->notify(&vq->vq);
 
@@ -324,6 +331,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
         ret = vq->data[i];
         detach_buf(vq, i);
         vq->last_used_idx++;
+        /* If we expect an interrupt for the next entry, tell host
+         * by writing event index and flush out the write before
+         * the read in the next get_buf call. */
+        if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
+                vring_used_event(&vq->vring) = vq->last_used_idx;
+                virtio_mb();
+        }
+
         END_USE(vq);
         return ret;
 }
@@ -345,7 +360,11 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 
         /* We optimistically turn back on interrupts, then check if there was
          * more to do. */
+        /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
+         * either clear the flags bit or point the event index at the next
+         * entry. Always do both to keep code simple. */
         vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+        vring_used_event(&vq->vring) = vq->last_used_idx;
         virtio_mb();
         if (unlikely(more_used(vq))) {
                 END_USE(vq);
@@ -438,6 +457,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
 #endif
 
         vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
+        vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
 
         /* No callback?  Tell other side not to bother us. */
         if (!callback)
@@ -472,6 +492,8 @@ void vring_transport_features(struct virtio_device *vdev)
                 switch (i) {
                 case VIRTIO_RING_F_INDIRECT_DESC:
                         break;
+                case VIRTIO_RING_F_EVENT_IDX:
+                        break;
                 default:
                         /* We don't understand this bit. */
                         clear_bit(i, vdev->features);