Commit a5c262c5 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by Rusty Russell
Browse files

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: default avatarMichael S. Tsirkin <>
Signed-off-by: default avatarRusty Russell <>
parent bf7035bf
......@@ -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;
/* Descriptors and available array need to be set before we expose the
* new available array entries. */
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 */
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. */
......@@ -324,6 +331,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
ret = vq->data[i];
detach_buf(vq, i);
/* 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;
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;
if (unlikely(more_used(vq))) {
......@@ -438,6 +457,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
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) {
/* We don't understand this bit. */
clear_bit(i, vdev->features);
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