linux-kernel - Re: [PATCH v4 2/5] x86/umwait: Initialize umwait control values

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Date:   Tue, 11 Jun 2019 10:50:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v4 2/5] x86/umwait: Initialize umwait control values

On Fri, Jun 07, 2019 at 03:00:34PM -0700, Fenghua Yu wrote:
> umwait or tpause allows processor to enter a light-weight
> power/performance optimized state (C0.1 state) or an improved
> power/performance optimized state (C0.2 state) for a period
> specified by the instruction or until the system time limit or until
> a store to the monitored address range in umwait.
> 
> IA32_UMWAIT_CONTROL MSR register allows kernel to enable/disable C0.2
> on the processor and set maximum time the processor can reside in
> C0.1 or C0.2.
> 
> By default C0.2 is enabled so the user wait instructions can enter the
> C0.2 state to save more power with slower wakeup time.
> 
> Default maximum umwait time is 100000 cycles. A later patch provides
> a sysfs interface to adjust this value.
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> Reviewed-by: Ashok Raj <ashok.raj@...el.com>
> Reviewed-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/msr-index.h |  4 +++
>  arch/x86/power/Makefile          |  1 +
>  arch/x86/power/umwait.c          | 56 ++++++++++++++++++++++++++++++++

Why is this in power/, this isn't in the least related to
suspend/hybernate. arch/x86/kernel/cpu/ might be a better place for
instruction support.