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

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

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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