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

[Jason Wang <jasowang@redhat.com>]

On 2016年11月12日 00:20, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 12:18:50PM +0800, Jason Wang wrote:
>>
>> 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@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.
>>> 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.
> What's wrong with what we have? It polls until value changes.
>

But as you said, this does not mean (in mergeable cases) we have enough 
space to store the packet.