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

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Date:   Fri, 20 May 2022 23:46:01 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Wyes Karny <wyes.karny@....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

On Fri, 2022-05-20 at 21:43 +0800, 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?
> 
> > >  
> > > -	if (!cpu_has(c, X86_FEATURE_MWAIT) ||
> > > boot_cpu_has_bug(X86_BUG_MONITOR))
> > > +	/* Monitor has a bug. Fallback to HALT */
> > > +	if (boot_cpu_has_bug(X86_BUG_MONITOR))
> > >  		return 0;
> > 
> > So, before, we pretty much just assume that all Intel CPUs with
> > MWAIT
> > should use MWAIT C1.
> > 
> > > -	return 1;
> > > +	cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &edx);
> > > +
> > > +	/*
> > > +	 * If MWAIT extensions are not available, it is safe to use
> > > MWAIT
> > > +	 * with EAX=0, ECX=0.
> > > +	 */
> > > +	if (!(ecx & CPUID5_ECX_EXTENSIONS_SUPPORTED))
> > > +		return 1;
> > > +
> > > +	/*
> > > +	 * If MWAIT extensions are available, there should be least one
> > > +	 * MWAIT C1 substate present.
> > > +	 */
> > > +	return (edx & MWAIT_C1_SUBSTATE_MASK);
> > >  }
> > 
> > So, I guess the "If MWAIT extensions are not available" check is
> > consistent with the "always use it on Intel" behavior.
> > 
> > But, this would change the behavior on Intel systems that both have
> > CPUID5_ECX_EXTENSIONS_SUPPORTED and do not set bits in
> > MWAIT_C1_SUBSTATE_MASK.
> > 
> > Is that a problem or an improvement?
> 
> At least Intel processors since Nehalem have MWAIT C1 support.
> For elder ones, need to confirm with Len.
> 
> When no bits set in MWAIT_C1_SUBSTATE_MASK, it means MWAIT C1 is not
> available for some reason, let me check if I can make this happen or
> not in real life.

I tried an Icelake server and a whiskeylake client, the supported
cstates in CPUID(5) edx doesn't change when a specific cstate or all
cstates are enabled/disabled in BIOS.

So there are always bits set in MWAIT_C1_SUBSTATE_MASK, and this patch
doesn't make any change to these Intel processors that have MWAIT C1
support.

thanks,
rui