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

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Date:   Fri, 6 May 2022 08:52:03 -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 5/6/22 02:42, Wyes Karny wrote:
> 
> So, if CPUID.05H:ECX[0] = 0, MWAIT extensions are not available (not allowed) and hence 
> it is safe to use MWAIT with EAX=0,ECX=0. Otherwise, we have to check CPUID.05H:EDX[bit 7:4]
> to get the number of C1 substates and this should be greater than equal to 1.

Ahh, I misread the comment.  I was confusing the CPUID leaf ECX data
with the use of ECX hints to MWAIT.

Could you add maybe a sentence or two more in that comment to help
clarify the situation?