[ltt-dev] [PATCH v2] cmm: provide lightweight smp_rmb/smp_wmb on PPC

[Mathieu Desnoyers]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> lwsync orders loads in cacheable memory with respect to other loads,
> and stores in cacheable memory with respect to other stores.  Use it
> to implement smp_rmb/smp_wmb.
> 
> The heavy-weight sync is still used for the "full" rmb/wmb operations,
> as well as for smp_mb.

[ Edit by Mathieu Desnoyers: rephrased the comments around the memory 
  barriers. ]


+/*
+ * Use sync for all cmm_mb/rmb/wmb barriers because lwsync does not
+ * preserve ordering of cacheable vs. non-cacheable accesses, so it
+ * should not be used to order with respect to MMIO operations.  An
+ * eieio+lwsync pair is also not enough for cmm_rmb, because it will
+ * order cacheable and non-cacheable memory operations separately---i.e.
+ * not the latter against the former.
+ */
+#define cmm_mb()         asm volatile("sync":::"memory")
+
+/*
+ * lwsync orders loads in cacheable memory with respect to other loads,
+ * and stores in cacheable memory with respect to other stores.
+ * Therefore, use it for barriers ordering accesses to cacheable memory
+ * only.
+ */
+#define cmm_smp_rmb()    asm volatile("lwsync":::"memory")
+#define cmm_smp_wmb()    asm volatile("lwsync":::"memory")


Merged, thanks!

Mathieu

> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> ---
>  urcu/arch/ppc.h |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
> 
> diff --git a/urcu/arch/ppc.h b/urcu/arch/ppc.h
> index a03d688..05f7db6 100644
> --- a/urcu/arch/ppc.h
> +++ b/urcu/arch/ppc.h
> @@ -32,7 +32,15 @@ extern "C" {
>  /* Include size of POWER5+ L3 cache lines: 256 bytes */
>  #define CAA_CACHE_LINE_SIZE	256
>  
> -#define cmm_mb()    asm volatile("sync":::"memory")
> +#define cmm_mb()         asm volatile("sync":::"memory")
> +
> +/* lwsync does not preserve ordering of cacheable vs. non-cacheable
> + * accesses, but it is good when MMIO is not in use.  An eieio+lwsync
> + * pair is also not enough for rmb, because it will order cacheable
> + * and non-cacheable memory operations separately---i.e. not the latter
> + * against the former.  */
> +#define cmm_smp_rmb()    asm volatile("lwsync":::"memory")
> +#define cmm_smp_wmb()    asm volatile("lwsync":::"memory")
>  
>  #define mftbl()						\
>  	({ 						\
> -- 
> 1.7.6
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com