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Subject[PATCHv2 03/14] 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>
---
drivers/virtio/virtio_ring.c | 26 ++++++++++++++++++++++++--
1 files changed, 24 insertions(+), 2 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cc2f73e..1d0f9be 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);
@@ -437,6 +456,7 @@ 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)
@@ -471,6 +491,8 @@ 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);
--
1.7.5.53.gc233e


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