linux-kernel - Re: [PATCH 4/7] x86/umwait_contro: Set global umwait maximum time limit and umwait C0.2 state

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Message-ID: <28650265-964f-caeb-5ebb-788bf86ee9e3@kernel.org>
Date:   Mon, 23 Jul 2018 18:41:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>
Cc:     Ashok Raj <ashok.raj@...el.com>, Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 4/7] x86/umwait_contro: Set global umwait maximum time
 limit and umwait C0.2 state

On 07/23/2018 05:55 AM, Fenghua Yu wrote:
> UMWAIT or TPAUSE called by user process makes processor to reside in
> a light-weight power/performance optimized state (C0.1 state) or an
> improved power/performance optimized state (C0.2 state).
> 
> IA32_UMWAIT_CONTROL MSR register allows OS to set global maximum umwait
> time and disable C0.2 on the processor.
> 
> The maximum time value in IA32_UMWAIT_CONTROL[31-2] is set as zero which
> means there is no global time limit for UMWAIT and TPAUSE instructions.
> Each process sets its own umwait maximum time as the instructions operand.
> We don't set a non-zero global umwait maximum time value to enforce user
> wait timeout because we couldn't find any usage for it.

Do you know what the instruction designers had in mind?  I assume they 
were thinking of *something*, but I'm seriously mystified by three things:

  - Why does CF work the way it does?  It seems like it would be 
genuinely useful for CF to indicate whether the in-register timeout has 
expired, but that's not what CF does.

  - Why does the global timeout apply even at CPL 0?

  - Why does the C0.2 control apply at CPL 0?

And I'm also a bit surprised that the instruction can't be turned off 
entirely for CPL 3.