linux-kernel - Re: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support

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Date:	Mon, 16 May 2011 21:31:23 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 4/6 net-next] vhost: vhost TX zero-copy support

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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