netdev - Re: [PATCH][VHOST] fix race with guest on multi-buffer used buffer updates

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Date:	Mon, 24 May 2010 09:27:15 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	kvm@...r.kernel.org, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org
Subject: Re: [PATCH][VHOST] fix race with guest on multi-buffer used buffer	updates

netdev-owner@...r.kernel.org 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@...hat.com> 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@...ibm.com>
> > > 
> > > 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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