linux-kernel - Re: [PATCH v3] x86/cpu/intel: Fix the constant

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Message-ID: <3093fd14-d57a-4fc6-9e15-d9ce8b075b30@intel.com>
Date: Thu, 31 Jul 2025 08:57:47 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Suchit Karunakaran <suchitkarunakaran@...il.com>, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
 darwi@...utronix.de, sohil.mehta@...el.com, peterz@...radead.org,
 ravi.bangoria@....com
Cc: skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] x86/cpu/intel: Fix the constant_tsc model check for
 Pentium 4s

On 7/29/25 21:26, Suchit Karunakaran wrote:
> The logic to synthesize constant_tsc for Pentium 4s (Family 15) is
> wrong. Since INTEL_P4_PRESCOTT is numerically greater than
> INTEL_P4_WILLAMETTE, the logic always results in false and never sets
> X86_FEATURE_CONSTANT_TSC for any Pentium 4 model.
> The error was introduced while replacing the x86_model check with a VFM
> one. The original check was as follows:
>         if ((c->x86 == 0xf && c->x86_model >= 0x03) ||
>                 (c->x86 == 0x6 && c->x86_model >= 0x0e))
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
> 
> Fix the logic to cover all Pentium 4 models from Prescott (model 3) to
> Cedarmill (model 6) which is the last model released in Family 15.

Could we have a slightly different changelog, please? The fact that the
logic results in the bit never getting set for P4's is IMNHO immaterial.
This looks like a plain and simple typo, not a logical error on the
patch author's part.

How about this as a changelog?

--

Pentium 4's which are INTEL_P4_PRESCOTT (mode 0x03) and later have a
constant TSC. This was correctly captured until fadb6f569b10
("x86/cpu/intel: Limit the non-architectural constant_tsc model
checks"). In that commit, the model was transposed from 0x03 to
INTEL_P4_WILLAMETTE, which is just plain wrong. That was presumably a
simple typo, probably just copying and pasting the wrong P4 model.

Fix the constant TSC logic to cover all later P4 models. End at
INTEL_P4_CEDARMILL which is the last P4 model.