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

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Date:   Tue, 3 Jan 2017 10:35:15 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>
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 Sat, Dec 31, 2016 at 3:09 AM, Borislav Petkov <bp@...en8.de> wrote:
> 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?

That's why I didn't type "volatile". :)

>
>> 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 :-)

Okay.  Anyway, this particular nit is minor and I'll shut up.

>
> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC