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

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Date:   Mon, 17 Jun 2019 13:46:19 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.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 v4 2/5] x86/umwait: Initialize umwait control values

On Tue, Jun 11, 2019 at 10:46:55PM +0200, Thomas Gleixner wrote:
> On Sun, 9 Jun 2019, Fenghua Yu wrote:
> 
> > On Sat, Jun 08, 2019 at 03:52:42PM -0700, Andy Lutomirski wrote:
> > > On Fri, Jun 7, 2019 at 3:10 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.
> > > 
> > > Sounds good, but:
> > > 
> > > > +#define MSR_IA32_UMWAIT_CONTROL_C02            BIT(0)
> > > 
> > > > +static u32 umwait_control_cached = 100000;
> > > 
> > > The code seems to disagree.
> > 
> > The definition of bit[0] is: C0.2 is disabled when bit[0]=1. So
> > 100000 means C0.2 is enabled (and max time is 100000).
> 
> which is totally non obvious. If you have to encode the control bit, then
> please make it explicit, i.e. mask out the disable bit in the initializer.

Is this right?

static u32 umwait_control_cached = 100000 & ~MSR_IA32_UMWAIT_CONTROL_C02_DISABLED;

Thanks.

-Fenghua