[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 04:45 PM, Marcelo Tosatti wrote:
> On Mon, Feb 22, 2010 at 04:23:32PM +0200, Avi Kivity wrote:
>    
> > On 02/22/2010 03:59 PM, Marcelo Tosatti wrote:
> >      
> > > Cc: "Michael S. Tsirkin"<address@hidden>
> > > Signed-off-by: Marcelo Tosatti<address@hidden>
> > >
> > > Index: qemu/kvm-all.c
> > > ===================================================================
> > > --- qemu.orig/kvm-all.c
> > > +++ qemu/kvm-all.c
> > > @@ -718,6 +718,9 @@ static int kvm_handle_io(uint16_t port,
> > >       return 1;
> > >   }
> > >
> > > +/* FIXME: arch dependant, x86 version */
> > > +#define smp_wmb()   asm volatile("" ::: "memory")
> > > +
> > >        
> > sfence?
> >      
> 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.  Perhaps it's only needed for WC writes 
(movnti and friends), but better be careful here.

> > what about other arches?
> >      
> They need to be fixed? PPC needs an instruction apparently.
>
> Is there any objection to including this patch?
>    

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.

--
error compiling committee.c: too many arguments to function