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Date: Fri, 19 Sep 2014 09:58:15 +0200 From: Borislav Petkov <bp@...en8.de> To: Paolo Bonzini <pbonzini@...hat.com> 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 On Thu, Sep 18, 2014 at 03:36:43PM +0200, Paolo Bonzini wrote: > We're talking about the case where the field is not reserved anymore and > we _know_ that the vendor has just decided to grow the bitfield that > precedes it. We're talking about the case where you assumed that a reserved bit is 0 which is an unsafe assumption, the least. > As soon as we know that the field is not reserved anymore, we > obviously rely on reserved bits being zero in older processors, and in > future processors from other vendors. Again, this is an unsafe assumption. > 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. 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. -- Regards/Gruss, Boris. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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