linux-kernel - Re: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync

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Date:   Wed, 22 Feb 2023 14:33:15 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Rohan McLure <rmclure@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb
 to lwsync

Sorry, sent the wrong patch!
Please ignore this one.
Sending the v2 in another email.

On Wed, Feb 22, 2023 at 02:31:12PM +0530, Kautuk Consul wrote:
> A link from ibm.com states:
> "Ensures that all instructions preceding the call to __lwsync
>  complete before any subsequent store instructions can be executed
>  on the processor that executed the function. Also, it ensures that
>  all load instructions preceding the call to __lwsync complete before
>  any subsequent load instructions can be executed on the processor
>  that executed the function. This allows you to synchronize between
>  multiple processors with minimal performance impact, as __lwsync
>  does not wait for confirmation from each processor."
> 
> Thats why smp_rmb() and smp_wmb() are defined to lwsync.
> But this same understanding applies to parallel pipeline
> execution on each PowerPC processor.
> So, use the lwsync instruction for rmb() and wmb() on the PPC
> architectures that support it.
> 
> Also removed some useless spaces.
> 
> Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/barrier.h | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index e80b2c0e9315..553f5a5d20bd 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -41,11 +41,17 @@
> 
>  /* The sub-arch has lwsync */
>  #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
> -#    define SMPWMB      LWSYNC
> +#undef rmb
> +#undef wmb
> +/* Redefine rmb() to lwsync. */
> +#define rmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> +/* Redefine wmb() to lwsync. */
> +#define wmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> +#define SMPWMB      LWSYNC
>  #elif defined(CONFIG_BOOKE)
> -#    define SMPWMB      mbar
> +#define SMPWMB      mbar
>  #else
> -#    define SMPWMB      eieio
> +#define SMPWMB      eieio
>  #endif
> 
>  /* clang defines this macro for a builtin, which will not work with runtime patching */
> -- 
> 2.31.1
>