linux-kernel - Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields

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Date:	Wed, 17 Sep 2014 16:06:01 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Nadav Amit <namit@...technion.ac.il>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields

On Wed, Sep 17, 2014 at 04:53:39PM +0300, Nadav Amit wrote:
> AFAIK backward compatibility is usually maintained in x86. I did not
> see in Intel SDM anything that says "this CPUID field means something
> for CPU X and something else for CPU Y". Anyhow, it is not different
> than bitmasks in this respect.

You still don't get my point: what are you going to do when
min_monitor_line_size needs to be 17 bits all of a sudden?

Currently, you simply do an if-else check before using the respective
mask and with your defined structs, you need to keep two versions:

union cpuid5_ebx_before_family_X {
       struct {
               unsigned int max_monitor_line_size:16;
               unsigned int reserved:16;
       } split;
       unsigned int full;
};

union cpuid5_ebx_after_family_X {
       struct {
               unsigned int max_monitor_line_size:17;
               unsigned int reserved:15;
       } split;
       unsigned int full;
};

> I don't understand what all the fuss is about.

And I don't understand why you're "fixing" code which doesn't need
fixing in the first place.

-- 
Regards/Gruss,
    Boris.
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