[Qemu-devel] Re: [patch uq/master 2/2] kvm-all.c: define smp_wmb and use it for coalesced mmio

[Avi Kivity]

On 02/22/2010 04:57 PM, Michael S. Tsirkin wrote:
>    
> > > There is no need (for this case). Older read cannot be reordered with
> > > write, writes are not reordered with other writes, writes by a single
> > > processor are observed in the same order by all processors.
> > >
> > >        
> > Well, Linux does use sfence.
> >      
> At least on 64 bit it doesnt.
>    

Right, I was looking at wmb(), not smp_wmb().

> > I imagine all arches need an instruction.  For reads as well.
> >
> > Note, gcc has a __sync_synchronize() builtin that compiles to mfence on
> > x86.  We might use that as a baseline for both rmb and wmb, and let each
> > arch override it incrementally.
> >      
> This it what my patch did. Note it only works well for recent gcc.
>    

Do you know how recent?

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error compiling committee.c: too many arguments to function