Re: [Xen-devel] [PATCH 32/34] xen/io: use __smp_XXX barriers

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 30 Dec 2015, Michael S. Tsirkin wrote:
> include/xen/interface/io/ring.h uses
> full memory barriers to communicate with the other side.
> 
> For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Switch to __smp_XXX barriers which serve this exact purpose.
> 
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>


>  include/xen/interface/io/ring.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
> index 7dc685b..46dfc65 100644
> --- a/include/xen/interface/io/ring.h
> +++ b/include/xen/interface/io/ring.h
> @@ -208,12 +208,12 @@ struct __name##_back_ring {                             
>                 \
>  
>  
>  #define RING_PUSH_REQUESTS(_r) do {                                  \
> -    wmb(); /* back sees requests /before/ updated producer index */  \
> +    __smp_wmb(); /* back sees requests /before/ updated producer index */    
> \
>      (_r)->sring->req_prod = (_r)->req_prod_pvt;                              
> \
>  } while (0)
>  
>  #define RING_PUSH_RESPONSES(_r) do {                                 \
> -    wmb(); /* front sees responses /before/ updated producer index */        
> \
> +    __smp_wmb(); /* front sees responses /before/ updated producer index */  
> \
>      (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                              
> \
>  } while (0)
>  
> @@ -250,9 +250,9 @@ struct __name##_back_ring {                               
>                 \
>  #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {                
> \
>      RING_IDX __old = (_r)->sring->req_prod;                          \
>      RING_IDX __new = (_r)->req_prod_pvt;                             \
> -    wmb(); /* back sees requests /before/ updated producer index */  \
> +    __smp_wmb(); /* back sees requests /before/ updated producer index */    
> \
>      (_r)->sring->req_prod = __new;                                   \
> -    mb(); /* back sees new requests /before/ we check req_event */   \
> +    __smp_mb(); /* back sees new requests /before/ we check req_event */     
> \
>      (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <                
> \
>                (RING_IDX)(__new - __old));                            \
>  } while (0)
> @@ -260,9 +260,9 @@ struct __name##_back_ring {                               
>                 \
>  #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {               
> \
>      RING_IDX __old = (_r)->sring->rsp_prod;                          \
>      RING_IDX __new = (_r)->rsp_prod_pvt;                             \
> -    wmb(); /* front sees responses /before/ updated producer index */        
> \
> +    __smp_wmb(); /* front sees responses /before/ updated producer index */  
> \
>      (_r)->sring->rsp_prod = __new;                                   \
> -    mb(); /* front sees new responses /before/ we check rsp_event */ \
> +    __smp_mb(); /* front sees new responses /before/ we check rsp_event */   
> \
>      (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <                
> \
>                (RING_IDX)(__new - __old));                            \
>  } while (0)
> @@ -271,7 +271,7 @@ struct __name##_back_ring {                               
>                 \
>      (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                        
> \
>      if (_work_to_do) break;                                          \
>      (_r)->sring->req_event = (_r)->req_cons + 1;                     \
> -    mb();                                                            \
> +    __smp_mb();                                                              
> \
>      (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                        
> \
>  } while (0)
>  
> @@ -279,7 +279,7 @@ struct __name##_back_ring {                               
>                 \
>      (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                       
> \
>      if (_work_to_do) break;                                          \
>      (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                     \
> -    mb();                                                            \
> +    __smp_mb();                                                              
> \
>      (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                       
> \
>  } while (0)
>  
> -- 
> MST
> 

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