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Subject[PATCHv2 11/14] virtio: don't delay avail index update
Update avail index immediately instead of upon kick:
for virtio-net RX this helps parallelism with the host.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/virtio/virtio_ring.c | 28 +++++++++++++++++++---------
1 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index eed5f29..8218fe6 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -89,7 +89,7 @@ struct vring_virtqueue
unsigned int num_free;
/* Head of free buffer list. */
unsigned int free_head;
- /* Number we've added since last sync. */
+ /* Number we've added since last kick. */
unsigned int num_added;

/* Last used index we've seen. */
@@ -174,6 +174,13 @@ int virtqueue_add_buf_gfp(struct virtqueue *_vq,

BUG_ON(data == NULL);

+ /* Prevent drivers from adding more than num bufs without a kick. */
+ if (vq->num_added == vq->vring.num) {
+ printk(KERN_ERR "gaaa!!!\n");
+ END_USE(vq);
+ return -ENOSPC;
+ }
+
/* If the host supports indirect descriptor tables, and we have multiple
* buffers, then go indirect. FIXME: tune this threshold */
if (vq->indirect && (out + in) > 1 && vq->num_free) {
@@ -227,8 +234,14 @@ add_head:

/* Put entry in available array (but don't update avail->idx until they
* do sync). FIXME: avoid modulus here? */
- avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
+ avail = vq->vring.avail->idx % vq->vring.num;
vq->vring.avail->ring[avail] = head;
+ vq->num_added++;
+
+ /* Descriptors and available array need to be set before we expose the
+ * new available array entries. */
+ virtio_wmb();
+ vq->vring.avail->idx++;

pr_debug("Added buffer head %i to %p\n", head, vq);
END_USE(vq);
@@ -242,17 +255,14 @@ 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();
-
- 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();

+ new = vq->vring.avail->idx;
+ old = new - vq->num_added;
+ vq->num_added = 0;
+
if (vq->event ?
vring_need_event(vring_avail_event(&vq->vring), new, old) :
!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
--
1.7.5.53.gc233e


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