netdev - Re: [PATCH 2/3] vhost: better detection of available buffers

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Date:   Fri, 11 Nov 2016 12:18:50 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] vhost: better detection of available buffers



On 2016年11月11日 11:41, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 10:18:37AM +0800, Jason Wang wrote:
>> >
>> >
>> >On 2016年11月10日 03:57, Michael S. Tsirkin wrote:
>>> > >On Wed, Nov 09, 2016 at 03:38:32PM +0800, Jason Wang wrote:
>>>> > > >We should use vq->last_avail_idx instead of vq->avail_idx in the
>>>> > > >checking of vhost_vq_avail_empty() since latter is the cached avail
>>>> > > >index from guest but we want to know if there's pending available
>>>> > > >buffers in the virtqueue.
>>>> > > >
>>>> > > >Signed-off-by: Jason Wang<jasowang@...hat.com>
>>> > >I'm not sure why is this patch here. Is it related to
>>> > >batching somehow?
>> >
>> >Yes, we need to know whether or not there's still buffers left in the
>> >virtqueue, so need to check last_avail_idx. Otherwise, we're checking if
>> >guest has submitted new buffers.
>> >
>>> > >
>>> > >
>>>> > > >---
>>>> > > >   drivers/vhost/vhost.c | 2 +-
>>>> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
>>>> > > >
>>>> > > >diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>>> > > >index c6f2d89..fdf4cdf 100644
>>>> > > >--- a/drivers/vhost/vhost.c
>>>> > > >+++ b/drivers/vhost/vhost.c
>>>> > > >@@ -2230,7 +2230,7 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>>> > > >   	if (r)
>>>> > > >   		return false;
>>>> > > >-	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
>>>> > > >+	return vhost16_to_cpu(vq, avail_idx) == vq->last_avail_idx;
>>>> > > >   }
>>>> > > >   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
>>> > >That might be OK for TX but it's probably wrong for RX
>>> > >where the fact that used != avail does not mean
>>> > >we have enough space to store the packet.
>> >
>> >Right, but it's no harm since it was just a hint, handle_rx() can handle
>> >this situation.
> Means busy polling will cause useless load on the CPU though.
>

Right, but,it's not easy to have 100% correct hint here. Needs more thought.