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Subject: Re: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support
From: Shirley Ma <mashirle@us.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: David Miller <davem@davemloft.net>, Eric Dumazet <eric.dumazet@gmail.com>,
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Date: Mon, 16 May 2011 13:56:54 -0700
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On Mon, 2011-05-16 at 23:45 +0300, Michael S. Tsirkin wrote:
> > +/* Since we need to keep the order of used_idx as avail_idx, it's
> possible that
> > + * DMA done not in order in lower device driver for some reason. To
> prevent
> > + * used_idx out of order, upend_idx is used to track avail_idx
> order, done_idx
> > + * is used to track used_idx order. Once lower device DMA done,
> then upend_idx
> > + * can move to done_idx.
> 
> Could you clarify this please? virtio explicitly allows out of order
> completion of requests. Does it simplify code that we try to keep
> used index updates in-order? Because if not, this is not
> really a requirement.

Hello Mike,

Based on my testing, vhost_add_used() must be in order from
vhost_get_vq_desc(). Otherwise, virtio_net ring seems get double
freed. I didn't spend time on debugging this.

in virtqueue_get_buf

        if (unlikely(!vq->data[i])) {
                BAD_RING(vq, "id %u is not a head!\n", i);
                return NULL;
        }

That's the reason I created the upend_idx and done_idx.

Thanks
Shirley