linux-kernel - Re: [PATCH v2 2/3] x86: Remove vendor checks from prefer_mwait_c1_over

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Date:   Thu, 5 May 2022 10:04:55 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     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 v2 2/3] x86: Remove vendor checks from
 prefer_mwait_c1_over_halt

On 5/5/22 04:01, Wyes Karny wrote:
> -	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;
> +
> +	/* Monitor has a bug. Fallback to HALT */
> +	if (boot_cpu_has_bug(X86_BUG_MONITOR))
>  		return 0;
>  
> -	if (!cpu_has(c, X86_FEATURE_MWAIT) || boot_cpu_has_bug(X86_BUG_MONITOR))
> +	if (c->cpuid_level < CPUID_MWAIT_LEAF)
>  		return 0;

First of all, thanks for all the detail in this new version of the patches!

In arch/x86/kernel/cpu/common.c, we have:

cpuid_dependent_features[] = {
        { X86_FEATURE_MWAIT,            0x00000005 },
	...

Shouldn't that clear X86_FEATURE_MWAIT on all systems without
CPUID_MWAIT_LEAF?  That would make the c->cpuid_level check above
unnecessary.

> +	/*
> +	 * If ECX doesn't have extended info about MWAIT,
> +	 * don't need to check substates.
> +	 */
> +	if (!(ecx & CPUID5_ECX_EXTENSIONS_SUPPORTED))
> +		return 1;

Could you explain a bit more about this?  I don't know this CPUID leaf
off the top of my head and the line after this is checking EDX.  It's
not apparent from this comment why "!ECX_EXTENSIONS_SUPPORTED" means
that MWAIT should be preferred.

> +	/* Check, whether at least 1 MWAIT C1 substate is present */
> +	return (edx & MWAIT_C1_SUBSTATE_MASK);