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

[Boqun Feng <boqun.feng@xxxxxxxxx>]

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
> 
> 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?

> -#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.

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

>       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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