Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers

[Paolo Bonzini]

On 09/06/2011 05:12 AM, David Gibson wrote:
> I'm not "fixing ppc".  I'm fixing a fundamental flaw in the protocol
> implementation._So far_  I've only observed the effects on ppc, but
> that doesn't mean they don't exist.

Actually Michael is right.  The implementation is correct on x86, though 
wrong anywhere else (perhaps s390?).  On those architectures you do not 
need rmb() and wmb().

So doing the __sync_synchronize() on ppc only as he proposed is wrong; 
but not doing it (and leaving only a compiler barrier) on x86 is 
correct.  See http://g.oswego.edu/dl/jmm/cookbook.html under 
"Multiprocessors".

Paolo