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Date: Thu, 10 May 2018 15:34:50 +0800
From: Jason Wang <jasowang@...hat.com>
To: Tiwei Bie <tiwei.bie@...el.com>, mst@...hat.com,
virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc: wexu@...hat.com
Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support
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
> _______________________________________________
> Virtualization mailing list
> Virtualization@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
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