On Mon, May 16, 2011 at 04:42:21PM +0930, Rusty Russell wrote:
On Sun, 15 May 2011 16:55:41 +0300, "Michael S. Tsirkin" [off-list ref] wrote:
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On Mon, May 09, 2011 at 02:03:26PM +0930, Rusty Russell wrote:
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On Wed, 4 May 2011 23:51:47 +0300, "Michael S. Tsirkin" [off-list ref] wrote:
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Use the new avail_event feature to reduce the number
of exits from the guest.
Figures here would be nice :)
You mean ASCII art in comments?
I mean benchmarks of some kind.
:)
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@@ -228,6 +237,12 @@ add_head:
* new available array entries. */
virtio_wmb();
vq->vring.avail->idx++;
+ /* If the driver never bothers to kick in a very long while,
+ * avail index might wrap around. If that happens, invalidate
+ * kicked_avail index we stored. TODO: make sure all drivers
+ * kick at least once in 2^16 and remove this. */
+ if (unlikely(vq->vring.avail->idx == vq->kicked_avail))
+ vq->kicked_avail_valid = true;
If they don't, they're already buggy. Simply do:
WARN_ON(vq->vring.avail->idx == vq->kicked_avail);
Hmm, but does it say that somewhere?
AFAICT it's a corollary of:
1) You have a finite ring of size <= 2^16.
2) You need to kick the other side once you've done some work.
Well one can imagine a driver doing:
while (virtqueue_get_buf()) {
virtqueue_add_buf()
}
virtqueue_kick()
which looks sensible (batch kicks) but might
process any number of bufs between kicks.
If we look at drivers closely enough, I think none
of them do the equivalent of the above, but not 100% sure.
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@@ -482,6 +517,8 @@ void vring_transport_features(struct virtio_device *vdev)
break;
case VIRTIO_RING_F_USED_EVENT_IDX:
break;
+ case VIRTIO_RING_F_AVAIL_EVENT_IDX:
+ break;
default:
/* We don't understand this bit. */
clear_bit(i, vdev->features);
Does this belong in a prior patch?
Thanks,
Rusty.
Well if we don't support the feature in the ring we should not
ack the feature, right?
Ah, you're right.
Thanks,
Rusty.