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

[Jason Wang]

On 2018å04æ25æ 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