linux-kernel - Re: [PATCH v2] x86/smpboot: Don't do mwait_play

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Message-ID: <20180426132829.GK15043@pd.tnic>
Date:   Thu, 26 Apr 2018 15:28:29 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Yazen Ghannam <Yazen.Ghannam@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/smpboot: Don't do mwait_play_dead() on AMD systems

On Tue, Apr 03, 2018 at 09:02:28AM -0500, Yazen Ghannam wrote:
> From: Yazen Ghannam <yazen.ghannam@....com>
> 
> Recent AMD systems support using MWAIT for C1 state. However, MWAIT will
> not allow deeper cstates than C1 on current systems.
> 
> With play_dead() we expect the OS to use the deepest state available.
> The deepest state available on AMD systems is reached through SystemIO
> or HALT. If MWAIT is available, we use it instead of the other methods,
> so we never reach the deepest state.
> 
> Don't try to use MWAIT to play_dead() on AMD systems. Instead, we'll use
> CPUIDLE to enter the deepest state advertised by firmware. If CPUIDLE is
> not available then we fallback to HALT.
> 
> Signed-off-by: Yazen Ghannam <yazen.ghannam@....com>
> ---
> Link:
> https://lkml.kernel.org/r/20180402183424.48222-1-Yazen.Ghannam@amd.com
> 
> v1->v2:
> * Drop comment in code.
> 
>  arch/x86/kernel/smpboot.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index ff99e2b6fc54..12599e55e040 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1536,6 +1536,8 @@ static inline void mwait_play_dead(void)
>  	void *mwait_ptr;
>  	int i;
>  
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> +		return;
>  	if (!this_cpu_has(X86_FEATURE_MWAIT))
>  		return;
>  	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
> -- 

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--