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Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support
To:     Tiwei Bie <tiwei.bie@intel.com>, mst@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Cc:     wexu@redhat.com, jfreimann@redhat.com
References: <20180425051550.24342-1-tiwei.bie@intel.com>
 <20180425051550.24342-4-tiwei.bie@intel.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <927f4478-5a81-31d4-ac69-f9ec26248591@redhat.com>
Date:   Thu, 10 May 2018 15:32:19 +0800
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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