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    SubjectRe: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support
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    On Tue, 2011-05-17 at 00:24 +0300, Michael S. Tsirkin wrote:
    > On Mon, May 16, 2011 at 01:56:54PM -0700, Shirley Ma wrote:
    > > 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.
    >
    > Double-freed or you get NULL below?

    More likely is NULL.

    > > 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;
    > > }
    >
    > Yes but i used here is the head that we read from the
    > ring, not the ring index itself.
    > i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id
    > we must complete any id only once, but in any order.

    It is in any order of ring id, but must be in the order of "head"
    returns from vhost_get_vq_desc(), any clue?


    Thanks
    Shirley



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