Messages in this thread | | | Subject | Re: [PATCH 2/3] vhost: better detection of available buffers | From | Jason Wang <> | Date | Fri, 11 Nov 2016 10:18:37 +0800 |
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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@redhat.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.
> > Maybe we should just rename this to vhost_vq_avail_unchanged > to clarify usage. >
Ok.
>> >> -- >> 2.7.4
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