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

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Message-ID: <59C811AB-F4DD-44EE-9645-E18C7DBE4C39@zytor.com>
Date: Thu, 08 May 2025 15:37:53 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.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 May 8, 2025 1:58:11 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>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

They did not. Non-Intel CPUs did, and do, report 0 for undefined levels. 

I believe even today Intel CPUs report the "last level" value for up to 0x7fffffff...