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

[Jason Wang]

On 2018å05æ10æ 15:32, Jason Wang wrote:
> On 2018å04æ25æ 13:15, Tiwei Bie wrote:
> > +ÂÂÂ /* We're using some buffers from the free list. */
> > +ÂÂÂ vq->vq.num_free -= descs_used;
> > +
> > +ÂÂÂ /* Update free pointer */
> > +ÂÂÂ if (indirect) {
> > +ÂÂÂÂÂÂÂ n = head + 1;
> > +ÂÂÂÂÂÂÂ if (n >= vq->vring_packed.num) {
> > +ÂÂÂÂÂÂÂÂÂÂÂ n = 0;
> > +ÂÂÂÂÂÂÂÂÂÂÂ vq->wrap_counter ^= 1;
> > +ÂÂÂÂÂÂÂ }
> > +ÂÂÂÂÂÂÂ vq->next_avail_idx = n;
> > +ÂÂÂ } else
> > +ÂÂÂÂÂÂÂ 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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