Re: [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version

[Jason Wang <jasowang@redhat.com>]

On 2020/6/10 下午7:05, Michael S. Tsirkin wrote:
>>> +EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
>>>    /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
>>>    void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
>>>    {
>>> +	unfetch_descs(vq);
>>>    	vq->last_avail_idx -= n;
>> So unfetch_descs() has decreased last_avail_idx.
>> Can we fix this by letting unfetch_descs() return the number and then we can
>> do:
>>
>> int d = unfetch_descs(vq);
>> vq->last_avail_idx -= (n > d) ? n - d: 0;
>>
>> Thanks
> That's intentional I think - we need both.


Yes, but:


>
> Unfetch_descs drops the descriptors in the cache that were
> *not returned to caller*  through get_vq_desc.
>
> vhost_discard_vq_desc drops the ones that were returned through get_vq_desc.
>
> Did I miss anything?

We could count some descriptors twice, consider the case e.g we only 
cache on descriptor:

fetch_descs()
     fetch_buf()
         last_avail_idx++;

Then we want do discard it:
vhost_discard_avail_buf(1)
     unfetch_descs()
         last_avail_idx--;
     last_avail_idx -= 1;

Thanks