linux-kernel - Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum

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Message-ID: <20161231110946.GA4789@nazgul.tnic>
Date:   Sat, 31 Dec 2016 12:09:46 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Junichi Nomura <j-nomura@...jp.nec.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a
 single datum

On Fri, Dec 30, 2016 at 06:13:24PM -0800, Andy Lutomirski wrote:
> Now you call it and ignore the return value and the compiler optimizes
> it out :)

Does it, really?

It is an inlined asm volatile. I checked all call sites and the CPUID
call is there. gcc 6 simply issues the CPUID and then later code
overwrites rAX. I.e., it looks ok to me.

Or what example scenario do you have in mind?

> Also, someone reading the code might scratch their head and
> wonder why you picked eax and not ebx, ecx, or edx.

We have comments for her/him :-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--