linux-kernel - Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state

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Date:   Mon, 17 Jun 2019 15:48:20 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 3/5] x86/umwait: Add sysfs interface to control umwait
 C0.2 state

On Sun, Jun 09, 2019 at 09:26:29PM -0700, Andy Lutomirski wrote:
> On Sun, Jun 9, 2019 at 9:14 PM Fenghua Yu <fenghua.yu@...el.com> wrote:
> >
> > On Sat, Jun 08, 2019 at 03:52:03PM -0700, Andy Lutomirski wrote:
> > > On Fri, Jun 7, 2019 at 3:10 PM Fenghua Yu <fenghua.yu@...el.com> wrote:
> > > >
> > > > C0.2 state in umwait and tpause instructions can be enabled or disabled
> > > > on a processor through IA32_UMWAIT_CONTROL MSR register.
> > > >
> > >
> > > > +static u32 get_umwait_control_c02(void)
> > > > +{
> > > > +       return umwait_control_cached & MSR_IA32_UMWAIT_CONTROL_C02;
> > > > +}
> > > > +
> > > > +static u32 get_umwait_control_max_time(void)
> > > > +{
> > > > +       return umwait_control_cached & MSR_IA32_UMWAIT_CONTROL_MAX_TIME;
> > > > +}
> > > > +
> > >
> > > I'm not convinced that these helpers make the code any more readable.
> >
> > The helpers reduce length of statements that call them. Otherwise, all of
> > the statements would be easily over 80 characters.
> >
> > Plus, each of the helpers is called multiple places in #0003 and #0004.
> > So the helpers make the patches smaller and cleaner.
> >
> 
> I was imagining things like:
> 
> umwait_control_cached &= ~MSR_IA32_UMWAIT_CONTROL_C02;
> if (whatever condition)
>   umwait_control_cached |= MSR_IA32_UMWAIT_CONTROL_C02;
> umwait_control_cached &= ~MSR_IA32_UMWAIT_CONTROL_MAX_TIME;
> umwait_control_cached |= new_max_time;

How about this statement?
With the helpers:
        umwait_control_cached = max_time | get_umwait_control_c02();
If there is no helpers, the above statement will need two statements:
	umwait_control_cached &= ~MSR_IA32_UMWAIT_CONTROL_MAX_TIME;
	umwait_control_cached |= max_time;

Another example:
With the helpers:
        if (umwait_control_c02 == get_umwait_control_c02())
If no helpers, the above statement will be long:
       if (umwait_control_c02 == (umwait_control_cached & MSR_IA32_UMWAIT_CONTROL_C02_DISABLED))

There are quite a few places like above examples.

The helpers can reduce the length of those long lines and make code more
readable and shorter, right?

Can I still keep the helpers?

Thanks.

-Fenghua