netdev - Re: [PATCH V4 net-next 1/3] vhost: better detection of available buffers

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Date:   Mon, 9 Jan 2017 10:59:16 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, stephen@...workplumber.org, wexu@...hat.com,
        stefanha@...hat.com
Subject: Re: [PATCH V4 net-next 1/3] vhost: better detection of available
 buffers



On 2017年01月07日 03:55, Michael S. Tsirkin wrote:
> On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote:
>> This patch tries to do several tweaks on vhost_vq_avail_empty() for a
>> better performance:
>>
>> - check cached avail index first which could avoid userspace memory access.
>> - using unlikely() for the failure of userspace access
>> - check vq->last_avail_idx instead of cached avail index as the last
>>    step.
>>
>> This patch is need for batching supports which needs to peek whether
>> or not there's still available buffers in the ring.
>>
>> Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/vhost/vhost.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index d643260..9f11838 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>   	__virtio16 avail_idx;
>>   	int r;
>>   
>> +	if (vq->avail_idx != vq->last_avail_idx)
>> +		return false;
>> +
>>   	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
>> -	if (r)
>> +	if (unlikely(r))
>>   		return false;
>> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>>   
>> -	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
>> +	return vq->avail_idx == vq->last_avail_idx;
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
> So again, this did not address the issue I pointed out in v1:
> if we have 1 buffer in RX queue and
> that is not enough to store the whole packet,
> vhost_vq_avail_empty returns false, then we re-read
> the descriptors again and again.
>
> You have saved a single index access but not the more expensive
> descriptor access.

Looks not, if I understand the code correctly, in this case, 
get_rx_bufs() will return zero, and we will try to enable rx kick and 
exit the loop.

Thanks