linux-kernel - Re: [PATCH] powerpc: Redefine HMT

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Message-ID: <878s0oby61.fsf@mpe.ellerman.id.au>
Date:   Thu, 26 Aug 2021 14:57:42 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: Redefine HMT_xxx macros as empty on PPC32

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> HMT_xxx macros are macros for adjusting thread priority
> (hardware multi-threading) are macros inherited from PPC64
> via commit 5f7c690728ac ("[PATCH] powerpc: Merged ppc_asm.h")
>
> Those instructions are pointless on PPC32, but some common
> fonctions like arch_cpu_idle() use them.
>
> So make them empty on PPC32 to avoid those instructions.

I guess we're pretty sure no 32-bit CPUs do anything with those.

e6500 can run in 32-bit mode, and is 2-way threaded AIUI, so it's
*possible* it could use them.

But I can't find any mention of those special nops in the e6500 core
manual. And actually it does have documentation about thread priority
registers, PPR32, TPRI0/1, but it says they're not used in e6500.

So I guess this seems safe, I'll pick it up.

cheers

> diff --git a/arch/powerpc/include/asm/vdso/processor.h b/arch/powerpc/include/asm/vdso/processor.h
> index e072577bc7c0..8d79f994b4aa 100644
> --- a/arch/powerpc/include/asm/vdso/processor.h
> +++ b/arch/powerpc/include/asm/vdso/processor.h
> @@ -5,12 +5,21 @@
>  #ifndef __ASSEMBLY__
>  
>  /* Macros for adjusting thread priority (hardware multi-threading) */
> +#ifdef CONFIG_PPC64
>  #define HMT_very_low()		asm volatile("or 31, 31, 31	# very low priority")
>  #define HMT_low()		asm volatile("or 1, 1, 1	# low priority")
>  #define HMT_medium_low()	asm volatile("or 6, 6, 6	# medium low priority")
>  #define HMT_medium()		asm volatile("or 2, 2, 2	# medium priority")
>  #define HMT_medium_high()	asm volatile("or 5, 5, 5	# medium high priority")
>  #define HMT_high()		asm volatile("or 3, 3, 3	# high priority")
> +#else
> +#define HMT_very_low()
> +#define HMT_low()
> +#define HMT_medium_low()
> +#define HMT_medium()
> +#define HMT_medium_high()
> +#define HMT_high()
> +#endif
>  
>  #ifdef CONFIG_PPC64
>  #define cpu_relax()	do { HMT_low(); HMT_medium(); barrier(); } while (0)
> -- 
> 2.25.0