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Date:	Fri, 19 Sep 2014 15:40:29 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Nadav Amit <namit@...technion.ac.il>,
	Ingo Molnar <mingo@...nel.org>,
	"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

Il 19/09/2014 09:58, Borislav Petkov ha scritto:
>> > The trivial example is feature bits like XSAVE. We query them all the
>> > time without checking the family when they were first introduced,
>> > don't we?
> The feature bits would obviously be 0 if features are not supported.

And similarly, Intel would not extend a bit from 16 to 17 bits if it
weren't zero on all older processors.

> However, even there
> 
> "16 - Reserved - Do not count on the value."
> 
> I'm quoting Intel's CPUID doc 241618-037 from 2011 (there might be a
> newer one though), the CPUID(1).ECX description.

Once that bit gets a meaning in newer processors, the same meaning will
work retroactively for existing processors.  That's just how CPUID is
used.  Nobody checks families before testing bits, Intel/AMD do not even
suggest that.

> Do you have a guarantee that this won't happen in the future and break
> all your fancy bitfields assumptions?

No guarantee, but were that to happen, I'd expect tar and feathers
spectacles around Intel's engineering headquarters.

Paolo
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