Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments

[Paul Brook <paul@codesourcery.com>]

On Wednesday 11 November 2009, Michael S. Tsirkin wrote:
> On Wed, Nov 11, 2009 at 02:16:00PM +0000, Paul Brook wrote:
> > On Wednesday 11 November 2009, Michael S. Tsirkin wrote:
> > > On Wed, Nov 11, 2009 at 01:45:35PM +0000, Paul Brook wrote:
> > > > If you don't need real barriers, then why does the kvm code have
> > > > them?
> > >
> > > We need real barriers but AFAIK kvm does not have them :(
> > > IOW: virtio is currently broken with kvm, and my patch did
> > > not fix this. The comment that I added says as much.
> >
> > So your code just makes the bug harder to reproduce? Doesn't sound like a
> > good thing to me.
> >
> > Paul
> 
> There are multiple bugs, I can't fix all of them,
> for all architectures, in one patch.

Maybe not, but the associated comment now appears to be bogus. Either we 
execute everything in lockstep (as the comment says, in which case no barriers 
are required), or we need proper SMP barriers.
Having a half-harted barrier that maybe works some of the time on some hosts, 
and is completely unnecessary in most cases, and doesn't match the comment is 
IMO significantly worse than what we had before.

Paul