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

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Message-ID: <CAO9wTFjaQZFg3U7eGjk+xXV6S-gKSAoV5sz7fqsuyYUkAMu_4g@mail.gmail.com>
Date: Thu, 31 Jul 2025 23:09:40 +0530
From: Suchit Karunakaran <suchitkarunakaran@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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, 
	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 Thu, 31 Jul 2025 at 21:29, Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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.

Yeah, I agree it's more of a typo than a logical error.