linux-kernel - Re: [PATCH v5] x86/cpu/intel: Fix the constant

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Message-ID: <CAO9wTFi9MHnzEw0p9QhhFOW_sk3mo_1yT3-YkjTYwN+spYUFWg@mail.gmail.com>
Date: Wed, 6 Aug 2025 21:08:23 +0530
From: Suchit Karunakaran <suchitkarunakaran@...il.com>
To: 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 v5] x86/cpu/intel: Fix the constant_tsc model check for
 Pentium 4

On Wed, 6 Aug 2025 at 21:04, Suchit Karunakaran
<suchitkarunakaran@...il.com> wrote:
>
> Pentium 4's which are INTEL_P4_PRESCOTT (model 0x03) and later have
> a constant TSC. This was correctly captured until commit fadb6f569b10
> ("x86/cpu/intel: Limit the non-architectural constant_tsc model checks").
>
> In that commit, an error was introduced while selecting the last P4
> model (0x06) as the upper bound. Model 0x06 was transposed 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 accurately corresponds to the last P4 model.
>
> Fixes: fadb6f569b10 ("x86/cpu/intel: Limit the non-architectural constant_tsc model checks")
> Cc: <stable@...r.kernel.org> # v6.15
> Signed-off-by: Suchit Karunakaran <suchitkarunakaran@...il.com>
> ---
> Changes since v4:
> - Updated the patch based on review suggestions
>
> Changes since v3:
> - Refined changelog
>
> Changes since v2:
> - Improved commit message
>
> Changes since v1:
> - Fixed incorrect logic
>
>  arch/x86/kernel/cpu/intel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 076eaa41b8c8..98ae4c37c93e 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -262,7 +262,7 @@ static void early_init_intel(struct cpuinfo_x86 *c)
>         if (c->x86_power & (1 << 8)) {
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>                 set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
> -       } else if ((c->x86_vfm >= INTEL_P4_PRESCOTT && c->x86_vfm <= INTEL_P4_WILLAMETTE) ||
> +       } else if ((c->x86_vfm >= INTEL_P4_PRESCOTT && c->x86_vfm <= INTEL_P4_CEDARMILL) ||
>                    (c->x86_vfm >= INTEL_CORE_YONAH  && c->x86_vfm <= INTEL_IVYBRIDGE)) {
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>         }
> --
> 2.50.1

Hi Sohil,
Could you please review the patch? I hope I haven't made any mistakes
this time, and if there are any, it was completely unintentional and I
apologise in advance.

Thanks,
Suchit