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Subject: Re: [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <be533349-817c-925b-43e4-899185d3fb1a@redhat.com>
Date:   Mon, 15 Jun 2020 10:43:42 +0800
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On 2020/6/11 下午5:06, Michael S. Tsirkin wrote:
> On Thu, Jun 11, 2020 at 11:02:57AM +0800, Jason Wang wrote:
>> 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
>
> I don't think that happens. vhost_discard_avail_buf(1) is only called
> after get vhost_get_avail_buf. vhost_get_avail_buf increments
> first_desc.  unfetch_descs only counts from first_desc to ndescs.
>
> If I'm wrong, could you show values of first_desc and ndescs in this
> scenario?


You're right, fetch_descriptor could not be called directly from the 
device code.

Thanks


>