linux-kernel - Re: [PATCH v1 02/26] x86/cpu: Sanitize CPUID(0x80000000) output

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Message-ID: <3af01720-6bd1-40cd-9292-2c35ae22296c@citrix.com>
Date: Thu, 8 May 2025 21:58:11 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: "H. Peter Anvin" <hpa@...or.com>, "Ahmed S. Darwish"
 <darwi@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
 John Ogness <john.ogness@...utronix.de>, x86@...nel.org,
 x86-cpuid@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 02/26] x86/cpu: Sanitize CPUID(0x80000000) output

On 08/05/2025 9:40 pm, H. Peter Anvin wrote:
> On May 7, 2025 1:50:48 AM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>> On 06/05/2025 6:04 am, Ahmed S. Darwish wrote:
>>> CPUID(0x80000000).EAX returns the max extended CPUID leaf available.  On
>>> x86-32 machines
>> How certain are you that it's all 32bit CPUs?  AIUI, it's an Intel
>> specific behaviour, not shared by other x86 vendors of the same era.
>>
>> ~Andrew
> All 64-bit machines require CPUID leaf 0x80000000.

Yes, but why's that relevant?

What I'm querying is the claim that all 32-bit machines behaved as Intel
did, and returned rubble for out-of-range leaves.

~Andrew