[ltt-dev] [URCU PATCH v3 2/2] cmm: do not generate code for smp_rmb/smp_wmb on x86_64, smp_rmb on i686

[Mathieu Desnoyers]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> Usually we can assume no accesses to write-combining memory occur,
> and also that there are no non-temporal load/stores (people would presumably
> write those with assembly or intrinsics and put appropriate lfence/sfence
> manually).  In this case rmb and wmb are no-ops on x86.  Define cmm_smp_rmb
> and cmm_smp_wmb to be the "common" operations, while leaving cmm_rmb
> and cmm_wmb in place for more sophisticated uses.
> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> ---
>  urcu/arch/x86.h |   25 +++++++++++++++++++++----
>  1 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/urcu/arch/x86.h b/urcu/arch/x86.h
> index 9e5411f..69f0416 100644
> --- a/urcu/arch/x86.h
> +++ b/urcu/arch/x86.h
> @@ -33,13 +33,30 @@ extern "C" {
>  
>  #ifdef CONFIG_RCU_HAVE_FENCE
>  #define cmm_mb()    asm volatile("mfence":::"memory")
> +
> +/*
> + * Define cmm_rmb/cmm_wmb to "strict" barriers that may be needed when
> + * using SSE or working with I/O areas.  cmm_smp_rmb/cmm_smp_wmb are
> + * only compiler barriers, which is enough for general use.
> + */
>  #define cmm_rmb()   asm volatile("lfence":::"memory")
>  #define cmm_wmb()   asm volatile("sfence"::: "memory")
> +
> +/*
> + * An empty cmm_smp_rmb() may not be enough on old PentiumPro multiprocessor
> + * systems, due to an erratum.  The Linux kernel says that "Even distro
> + * kernels should think twice before enabling this", but for now let's
> + * be conservative and leave the full barrier on 32-bit processors.
> + */
> +#define cmm_smp_rmb() cmm_barrier()

This comment does not match the #if /#else branch of the code it
discusses. It should be moved to the #else branch.

> +
> +/*
> + * IDT WinChip supports weak store ordering, and the kernel may enable it
> + * under our feet; cmm_smp_wmb() ceases to be a nop for these processors.
> + * However, this never happens on any processor that has *fence instructions.

Same for the comment about IDT WinChip.

Thanks!

Mathieu

> + */
> +#define cmm_smp_wmb() cmm_barrier()
>  #else
> -/*
> - * Some non-Intel clones support out of order store. cmm_wmb() ceases to be a
> - * nop for these.
> - */
>  #define cmm_mb()    asm volatile("lock; addl $0,0(%%esp)":::"memory")
>  #define cmm_rmb()   asm volatile("lock; addl $0,0(%%esp)":::"memory")
>  #define cmm_wmb()   asm volatile("lock; addl $0,0(%%esp)"::: "memory")
> -- 
> 1.7.6
> 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com