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

[Paul E. McKenney]

On Thu, Sep 22, 2011 at 10:42:52AM +0200, Paolo Bonzini 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.
> 
> 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")

This works for recent Power hardware, and I see no reason to care about
stuff old enough to lack lwsync.  I must defer to others on embedded
PowerPC.

							Thanx, Paul