Re: [Qemu-devel] [PATCHv2 1/3] virtio: add missing mb() on notification

[David Gibson]

On Tue, Apr 24, 2012 at 03:46:25PM +0200, Paolo Bonzini wrote:
> Il 23/04/2012 15:19, Michael S. Tsirkin ha scritto:
[snip]
> >  #elif defined(_ARCH_PPC)
> >  
> >  /*
> > - * We use an eieio() for a wmb() on powerpc.  This assumes we don't
> > + * We use an eieio() for wmb() and mb() on powerpc.  This assumes we don't
> >   * need to order cacheable and non-cacheable stores with respect to
> >   * each other
> >   */
> >  #define smp_wmb()   asm volatile("eieio" ::: "memory")
> > +#define smp_mb()   asm volatile("eieio" ::: "memory")
> 
> smp_mb() is hwsync under PPC, but I would just trust GCC.

I assume you mean 'lwsync', no such thing as 'hwsync', afaik.  And I
assume you're talking about the kernel here.

So, the situation in qemu is different from in the kernel, because as
far as I know there's no case in qemu where we need to synchronize
cacheable stores with non-cacheable (I/O) stores.  eieio won't do
that, but it will order cacheable stores w.r.t. other cacheable
stores.  I think __sync_synchronize() will be a 'sync', the most
heavyweight memory barrier on ppc.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson