linux-kernel - Re: [x86/cpuid] Question regarding historic leaf 0x80000000 code

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Message-ID: <Z9xy04RL3Uww9DYy@lx-t490>
Date: Thu, 20 Mar 2025 20:56:03 +0100
From: "Ahmed S. Darwish" <darwi@...utronix.de>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	John Ogness <john.ogness@...utronix.de>, x86@...nel.org,
	x86-cpuid@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [x86/cpuid] Question regarding historic leaf 0x80000000 code

On Thu, 20 Mar 2025, Andrew Cooper wrote:
>
> The problem is that for an out-of-range leaf, Intel returns the data
> from the maximum in-range leaf, rather than zeroes.
>
> On pre-AMD64-capable Intel CPUs, you'll get what amounts to junk in a
> query for leaf 0x80000000, where it's probably leaf 5 or so.
>
> Checking for the upper half of the output matching the input is a way of
> distinguishing Intel behaviour from all the other vendors.
>

Thanks a lot Andrew for all your help!

Then I'll mention that, in a comment, on top of the new CPUID scanner's
leaf 0x8000000 code.

Kind regards,

--
Ahmed S. Darwish
Linutronix GmbH