netdev - Re: [RFC v3 3/5] virtio_ring: add packed ring support

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Date:   Thu, 10 May 2018 17:49:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com
Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support



On 2018年05月10日 16:56, Tiwei Bie wrote:
> On Thu, May 10, 2018 at 03:34:50PM +0800, Jason Wang wrote:
>> 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
> Nice catch! Thanks a lot!
> I'll implement an ID allocator.
>
> Best regards,
> Tiwei Bie

Sounds good.

Another similar issue is detac_buf_packed(). It did:

         for (j = 0; j < vq->desc_state[head].num; j++) {
                 desc = &vq->vring_packed.desc[i];
                 vring_unmap_one_packed(vq, desc);
                 i++;
                 if (i >= vq->vring_packed.num)
                         i = 0;
         }

This probably won't work for out of order too and according to the spec:

"""
Driver needs to keep track of the size of the list corresponding to each
buffer ID, to be able to skip to where the next used descriptor is 
written by the device.
"""

Looks like we should not depend on the descriptor ring.

Thanks