[ltt-dev] [URCU PATCH] caa: do not generate code for rmb/wmb on x86_64, rmb on i686

[Mathieu Desnoyers]

Hi Paolo,

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> In userspace 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).  So rmb and wmb are no-ops on x86.

What about memory barriers for DMA with devices ? For these, we might
want to define cmm_wmb/rmb and cmm_smp_wmb/rmb differently (keep the
fences for DMA accesses).

So people who want to use memory barriers for non-temporal load/stores
could use the cmm_wmb/rmb variants too.

> 
> But IDT chips are an exception, so keep wmb on 32-bit and document better
> the rationale.
> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> ---
>  urcu/arch/x86.h |   14 +++++++++-----
>  1 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/urcu/arch/x86.h b/urcu/arch/x86.h
> index 9e5411f..d25f13d 100644
> --- a/urcu/arch/x86.h
> +++ b/urcu/arch/x86.h
> @@ -33,15 +33,19 @@ extern "C" {
>  
>  #ifdef CONFIG_RCU_HAVE_FENCE
>  #define cmm_mb()    asm volatile("mfence":::"memory")
> -#define cmm_rmb()   asm volatile("lfence":::"memory")
> -#define cmm_wmb()   asm volatile("sfence"::: "memory")
> +#define cmm_rmb()   asm volatile("":::"memory")
> +#define cmm_wmb()   asm volatile(""::: "memory")
>  #else
>  /*
> - * Some non-Intel clones support out of order store. cmm_wmb() ceases to be a
> - * nop for these.
> + * IDT WinChip supports weak store ordering, and the kernel may enable it
> + * under our feet; cmm_wmb() ceases to be a nop for these processors.
> + *
> + * The same would hold for cmm_rmb() on some old PentiumPro multiprocessor
> + * systems that have an errata, but the Linux kernel says that "Even distro
> + * kernels should think twice before enabling this".

Maybe we should have configure options --without-x86-ppro-support and
--without-x86-idt-winchip-support for this ? I really want the default
to be bullet-proof. So deactivating support for these specific
architectures on a per-distro basis would make more sense.

Thanks,

Mathieu

>   */
>  #define cmm_mb()    asm volatile("lock; addl $0,0(%%esp)":::"memory")
> -#define cmm_rmb()   asm volatile("lock; addl $0,0(%%esp)":::"memory")
> +#define cmm_rmb()   asm volatile("":::"memory")
>  #define cmm_wmb()   asm volatile("lock; addl $0,0(%%esp)"::: "memory")
>  #endif

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