linux-kernel - Re: [PATCH v3 2/3] x86: Remove vendor checks from prefer_mwait_c1_over

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Message-ID: <17fd6bd4a159cc23f20e8e3524f0cdcb871b434d.camel@intel.com>
Date:   Wed, 25 May 2022 15:20:48 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Wyes Karny <wyes.karny@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     Lewis.Carroll@....com, Mario.Limonciello@....com,
        gautham.shenoy@....com, Ananth.Narayan@....com, bharata@....com,
        len.brown@...el.com, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, chang.seok.bae@...el.com,
        keescook@...omium.org, metze@...ba.org, zhengqi.arch@...edance.com,
        mark.rutland@....com, puwen@...on.cn, rafael.j.wysocki@...el.com,
        andrew.cooper3@...rix.com, jing2.liu@...el.com,
        jmattson@...gle.com, pawan.kumar.gupta@...ux.intel.com
Subject: Re: [PATCH v3 2/3] x86: Remove vendor checks from
 prefer_mwait_c1_over_halt

Hi, Wyes,

On Mon, 2022-05-23 at 21:19 +0530, Wyes Karny wrote:
> Hello Rui,
> 
> On 5/20/2022 7:13 PM, Zhang Rui wrote:
> > On Thu, 2022-05-19 at 09:00 -0700, Dave Hansen wrote:
> > > On 5/10/22 03:18, Wyes Karny wrote:
> > > >  static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86
> > > > *c)
> > > >  {
> > > > +	u32 eax, ebx, ecx, edx;
> > > > +
> > > >  	/* User has disallowed the use of MWAIT. Fallback to
> > > > HALT */
> > > >  	if (boot_option_idle_override == IDLE_NOMWAIT)
> > > >  		return 0;
> > > >  
> > > > -	if (c->x86_vendor != X86_VENDOR_INTEL)
> > > > +	/* MWAIT is not supported on this platform. Fallback to
> > > > HALT */
> > > > +	if (!cpu_has(c, X86_FEATURE_MWAIT))
> > > >  		return 0;
> > 
> > I'm new to x86 code, a dumb question, what about the other vendors?
> > with this patch, prefer_mwait_c1_over_halt() can return 1 for other
> > vendors as well?
> 
> This decision tree is based on cpuid features, so if the processor
> advertises MWAIT C1 support we would be choosing MWAIT.
> If any vendor wants to avoid choosing MWAIT (even cpuid shows MWAIT
> C1
> support), they can explicitly mention. Will add exceptions form them.
> 
That's my concern.

This patch indeed changes the behaviors for vendors other than Intel
and AMD.
I don't know the impact of this, but just point out a behavior change
caused by this patch.

thanks,
rui