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>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index b0043fb26a4d5dd8fcc9a2b7b66a18b3b44053b0..a5fadc40c448959a8f766758d7bf976988b78ba3 100644 (file)
--- 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);