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Date: Tue, 28 Oct 2014 07:32:28 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20141028053228.GD18689@redhat.com>
References: <1414462382-24264-1-git-send-email-wu.wubin@huawei.com>
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In-Reply-To: <1414462382-24264-1-git-send-email-wu.wubin@huawei.com>
Subject: Re: [Qemu-devel] [PATCH] hw/virtio/virtio.c: fix the
	vring_avail_event error
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To: Bin Wu <wu.wubin@huawei.com>
Cc: pbonzini@redhat.com, famz@redhat.com, qemu-devel@nongnu.org

On Tue, Oct 28, 2014 at 02:13:02AM +0000, Bin Wu wrote:
> The event idx in virtio is an effective way to reduce the number of
> interrupts and exits of the guest. When the guest puts an request
> into the virtio ring, it doesn't exit immediately to inform the
> backend. Instead, the guest checks the "avail" event idx to determine
> the notification.
> 
> In virtqueue_pop, when a request is poped, the current avail event
> idx should be set to the number of vq->last_avail_idx.
> 
> Signed-off-by: Bin Wu <wu.wubin@huawei.com>

Does this fix some observable bug? Improve efficiency for some workload?

> ---
>  hw/virtio/virtio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 2c236bf..013979a 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -469,7 +469,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
>  
>      i = head = virtqueue_get_head(vq, vq->last_avail_idx++);
>      if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
> -        vring_avail_event(vq, vring_avail_idx(vq));
> +        vring_avail_event(vq, vq->last_avail_idx);
>      }
>  
>      if (vring_desc_flags(vdev, desc_pa, i) & VRING_DESC_F_INDIRECT) {
> -- 
> 1.7.12.4