linux-kernel - Re: [PATCH v3 2/5] x86/umwait: Initialize umwait control values

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Date:   Thu, 30 May 2019 14:05:36 -0700
From:   Andy Lutomirski <luto@...nel.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>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        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 v3 2/5] x86/umwait: Initialize umwait control values

On Fri, May 24, 2019 at 5:05 PM Fenghua Yu <fenghua.yu@...el.com> 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.

Reviewed-by: Andy Lutomirski <luto@...nel.org>

with the caveat that we should really clean up our CPU init code to
have a function like cpu_prepare_for_user_code() that is called on all
CPUs after every boot, resume, etc before running user code.  This
would subsume syscall_init().

--Andy