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virtio: expose added descriptors immediately.
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. In the unusual (but now possible) case where a driver does add_buf() and get_buf() without doing a kick, we do need to insert one before our counter wraps. Otherwise we could wrap num_added, and later on not realize that we have passed the marker where we should have kicked. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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@ -251,9 +251,20 @@ add_head:
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/* Put entry in available array (but don't update avail->idx until they
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* do sync). */
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avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
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avail = (vq->vring.avail->idx & (vq->vring.num-1));
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vq->vring.avail->ring[avail] = head;
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/* Descriptors and available array need to be set before we expose the
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* new available array entries. */
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virtio_wmb(vq);
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vq->vring.avail->idx++;
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vq->num_added++;
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/* This is very unlikely, but theoretically possible. Kick
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* just in case. */
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if (unlikely(vq->num_added == (1 << 16) - 1))
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virtqueue_kick(_vq);
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pr_debug("Added buffer head %i to %p\n", head, vq);
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END_USE(vq);
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@ -283,13 +294,10 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
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* new available array entries. */
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virtio_wmb(vq);
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old = vq->vring.avail->idx;
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new = vq->vring.avail->idx = old + vq->num_added;
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old = vq->vring.avail->idx - vq->num_added;
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new = vq->vring.avail->idx;
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vq->num_added = 0;
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/* Need to update avail index before checking if we should notify */
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virtio_mb(vq);
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if (vq->event) {
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needs_kick = vring_need_event(vring_avail_event(&vq->vring),
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new, old);
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