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

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Date:   Tue, 3 Apr 2018 13:03:52 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yazen Ghannam <Yazen.Ghannam@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, bp@...e.de
Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD systems


* Yazen Ghannam <Yazen.Ghannam@....com> 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>
> ---
>  arch/x86/kernel/smpboot.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index ff99e2b6fc54..67cf00b25f83 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1536,6 +1536,9 @@ static inline void mwait_play_dead(void)
>  	void *mwait_ptr;
>  	int i;
>  
> +	/* Don't try native MWAIT on AMD. Stick to CPUIDLE and HALT. */
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> +		return;

The comment should mainly explain the 'why is this done', not the 'what is done' 
which is pretty obvious from the code ...

Thanks,

	Ingo