lkml.org 
[lkml]   [2011]   [Nov]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Subject[PATCH 5 of 5] virtio: expose added descriptors immediately
Date
From
A virtio driver does virtqueue_add_buf() multiple times before finally
calling virtqueue_kick(); previously we only exposed the added buffers
in the virtqueue_kick() call. This means we don't need a memory
barrier in virtqueue_add_buf(), but it reduces concurrency as the
device (ie. host) can't see the buffers until the kick.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++---------------
1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -227,9 +227,15 @@ add_head:

/* Put entry in available array (but don't update avail->idx until they
* do sync). */
- avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
+ avail = (vq->vring.avail->idx & (vq->vring.num-1));
vq->vring.avail->ring[avail] = head;

+ /* 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++;
+
pr_debug("Added buffer head %i to %p\n", head, vq);
END_USE(vq);

@@ -248,13 +254,10 @@ bool virtqueue_kick_prepare(struct virtq
* new available array entries. */
virtio_wmb();

- old = vq->vring.avail->idx;
- new = vq->vring.avail->idx = old + vq->num_added;
+ old = vq->vring.avail->idx - vq->num_added;
+ new = vq->vring.avail->idx;
vq->num_added = 0;

- /* Need to update avail index before checking if we should notify */
- virtio_mb();
-
if (vq->event) {
needs_kick = vring_need_event(vring_avail_event(&vq->vring),
new, old);



\
 
 \ /
  Last update: 2011-11-03 08:47    [W:0.112 / U:23.456 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site