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Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support
From:   Jason Wang <jasowang@redhat.com>
To:     Tiwei Bie <tiwei.bie@intel.com>, mst@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Cc:     wexu@redhat.com
References: <20180425051550.24342-1-tiwei.bie@intel.com>
 <20180425051550.24342-4-tiwei.bie@intel.com>
 <927f4478-5a81-31d4-ac69-f9ec26248591@redhat.com>
Message-ID: <5885acac-e9e3-3abf-b6a2-7347f4d55be2@redhat.com>
Date:   Thu, 10 May 2018 15:34:50 +0800
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On 2018年05月10日 15:32, Jason Wang wrote:
>
>
> On 2018年04月25日 13:15, Tiwei Bie wrote:
>> +    /* We're using some buffers from the free list. */
>> +    vq->vq.num_free -= descs_used;
>> +
>> +    /* Update free pointer */
>> +    if (indirect) {
>> +        n = head + 1;
>> +        if (n >= vq->vring_packed.num) {
>> +            n = 0;
>> +            vq->wrap_counter ^= 1;
>> +        }
>> +        vq->next_avail_idx = n;
>> +    } else
>> +        vq->next_avail_idx = i;
>
> During testing zerocopy (out of order completion), I found driver may 
> submit two identical buffer id to vhost. So the above code may not 
> work well.
>
> Consider the case that driver adds 3 buffer and virtqueue size is 8.
>
> a) id = 0,count = 2,next_avail = 2
>
> b) id = 2,count = 4,next_avail = 2

next_avail should be 6 here.

>
> c) id = 4,count = 2,next_avail = 0
>

id should be 6 here.

Thanks

> if packet b is done before packet a, driver may think buffer id 0 is 
> available and try to use it if even if the real buffer 0 was not done.
>
> Thanks
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