netdev-owner@xxxxxxxxxxxxxxx wrote on 05/24/2010 09:13:51 AM:
> On Mon, May 24, 2010 at 08:52:40AM -0700, David Stevens wrote:
> > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote on 05/24/2010 03:17:10 AM:
> >
> > > On Thu, May 20, 2010 at 09:58:06AM -0700, David L Stevens wrote:
> > > > [for Michael Tsirkin's vhost development git tree]
> > > >
> > > > This patch fixes a race between guest and host when
> > > > adding used buffers wraps the ring. Without it, guests
> > > > can see partial packets before num_buffers is set in
> > > > the vnet header.
> > > >
> > > > Signed-off-by: David L Stevens <dlstevens@xxxxxxxxxx>
> > >
> > > Could you please explain what the race is?
> >
> > Sure. The pre-patch code in the ring-wrap case
> > does this:
> >
> > add part1 bufs
> > update used index
> > add part2 bufs
> > update used index
> >
> > After we update the used index for part1, the part1
> > buffers are available to the guest. If the guest is
> > consuming at that point, it can process the partial
> > packet before the rest of the packet is there. In that
> > case, num_buffers will be greater than the number of
> > buffers available to the guest and it'll drop the
> > packet with a framing error. I was seeing 2 or 3 framing
> > errors every 100 million packets or so pre-patch, none
> > post-patch.
> > Actually, the second sentence is incorrect in the
> > original description-- num_buffers is up to date when
> > the guest sees it, but the used index is not.
> >
> > +-DLS
>
> so this happens always - what does wrap-around refer to?
The 2-part update only happens when a packet spans
the end/beginning of the vring (the wrap). The framing error only
happens if the guest sees the vring-wrapping packets
before the second used-index write (the race).
So, the framing error doesn't happen always--it's
pretty rare. But with the patch, it never happens.
+-DLS
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