Re: [Xen-devel] [PATCH v2 15/32] powerpc: define __smp_xxx

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, Jan 05, 2016 at 09:36:55AM +0800, Boqun Feng wrote:
> Hi Michael,
> 
> On Thu, Dec 31, 2015 at 09:07:42PM +0200, Michael S. Tsirkin wrote:
> > This defines __smp_xxx barriers for powerpc
> > for use by virtualization.
> > 
> > smp_xxx barriers are removed as they are
> > defined correctly by asm-generic/barriers.h

I think this is the part that was missed in review.

> > This reduces the amount of arch-specific boiler-plate code.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >  arch/powerpc/include/asm/barrier.h | 24 ++++++++----------------
> >  1 file changed, 8 insertions(+), 16 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/barrier.h 
> > b/arch/powerpc/include/asm/barrier.h
> > index 980ad0c..c0deafc 100644
> > --- a/arch/powerpc/include/asm/barrier.h
> > +++ b/arch/powerpc/include/asm/barrier.h
> > @@ -44,19 +44,11 @@
> >  #define dma_rmb()  __lwsync()
> >  #define dma_wmb()  __asm__ __volatile__ (stringify_in_c(SMPWMB) : : 
> > :"memory")
> >  
> > -#ifdef CONFIG_SMP
> > -#define smp_lwsync()       __lwsync()
> > +#define __smp_lwsync()     __lwsync()
> >  
> 
> so __smp_lwsync() is always mapped to lwsync, right?

Yes.

> > -#define smp_mb()   mb()
> > -#define smp_rmb()  __lwsync()
> > -#define smp_wmb()  __asm__ __volatile__ (stringify_in_c(SMPWMB) : : 
> > :"memory")
> > -#else
> > -#define smp_lwsync()       barrier()
> > -
> > -#define smp_mb()   barrier()
> > -#define smp_rmb()  barrier()
> > -#define smp_wmb()  barrier()
> > -#endif /* CONFIG_SMP */
> > +#define __smp_mb() mb()
> > +#define __smp_rmb()        __lwsync()
> > +#define __smp_wmb()        __asm__ __volatile__ (stringify_in_c(SMPWMB) : 
> > : :"memory")
> >  
> >  /*
> >   * This is a barrier which prevents following instructions from being
> > @@ -67,18 +59,18 @@
> >  #define data_barrier(x)    \
> >     asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
> >  
> > -#define smp_store_release(p, v)                                            
> > \
> > +#define __smp_store_release(p, v)                                          
> > \
> >  do {                                                                       
> > \
> >     compiletime_assert_atomic_type(*p);                             \
> > -   smp_lwsync();                                                   \
> > +   __smp_lwsync();                                                 \
> 
> , therefore this will emit an lwsync no matter SMP or UP.

Absolutely. But smp_store_release (without __) will not.

Please note I did test this: for ppc code before and after
this patch generates exactly the same binary on SMP and UP.


> Another thing is that smp_lwsync() may have a third user(other than
> smp_load_acquire() and smp_store_release()):
> 
> http://article.gmane.org/gmane.linux.ports.ppc.embedded/89877
> 
> I'm OK to change my patch accordingly, but do we really want
> smp_lwsync() get involved in this cleanup? If I understand you
> correctly, this cleanup focuses on external API like smp_{r,w,}mb(),
> while smp_lwsync() is internal to PPC.
> 
> Regards,
> Boqun

I think you missed the leading ___ :)

smp_store_release is external and it needs __smp_lwsync as
defined here.

I can duplicate some code and have smp_lwsync *not* call __smp_lwsync
but why do this? Still, if you prefer it this way,
please let me know.

> >     WRITE_ONCE(*p, v);                                              \
> >  } while (0)
> >  
> > -#define smp_load_acquire(p)                                                
> > \
> > +#define __smp_load_acquire(p)                                              
> > \
> >  ({                                                                 \
> >     typeof(*p) ___p1 = READ_ONCE(*p);                               \
> >     compiletime_assert_atomic_type(*p);                             \
> > -   smp_lwsync();                                                   \
> > +   __smp_lwsync();                                                 \
> >     ___p1;                                                          \
> >  })
> >  
> > -- 
> > MST
> > 
> > --
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