Re: [PATCH 09/18] virtio: use avail_event index

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.