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Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_m


From: Marc-André Lureau
Subject: Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table.
Date: Sat, 13 Aug 2016 02:13:46 -0400 (EDT)

Hi

----- Original Message -----
> On Fri, Aug 12, 2016 at 11:54:54AM -0400, Marc-André Lureau wrote:
> > Hi
> > 
> > ----- Original Message -----
> > > On Fri, Aug 12, 2016 at 03:20:56AM -0400, Marc-André Lureau wrote:
> > > > Hi
> > > > 
> > > > ----- Original Message -----
> > > > > sent a follow-up response to GET_FEATURES), I am now wondering if
> > > > > this
> > > > > patch
> > > > > may break existing vhost applications too ? If so, reverting it
> > > > > possibly
> > > > > better.
> > > > > What confuses me is why it doesn’t fail all the time, but only about
> > > > > 20%
> > > > > to
> > > > > 30% time as Fam reports.
> > > > > 
> > > > > Thoughts : Michael, Fam, MarcAndre ?
> > > > 
> > > > Indeed, I didn't ack that patch in the first place for that kind of
> > > > reasons, so I would revert it.
> > > > 
> > > > thanks
> > > 
> > > I guess that's the safest thing to do for 2.7.
> > > At least that's not any worse than 2.6.
> > > I still think it's a good idea long term and test should be fixed,
> > > but let's revert for now.
> > > 
> > 
> > What about other backends that may have similar expectations from the
> > protocol.
> > 
> > This patch is a hack, there is no reason to have it upstream.
> 
> The reason is to avoid crashes with existing backends.

Which backend? I had a similar issue, it wasn't about crashes, and Prerna 
didn't mention crashes either, but anyway there is not guarantee that adding a 
GET_FEATURES message will solve it...


> > The solution is provided with the REPLY_ACK patch.
> 
> It needs a backend update though.
> 
> But the issue is old, it's not a regression. I think we lose nothing
> by pushing the work-around out until after 2.7.
> 
> --
> MST
> 



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