Re: [PATCH v3] x86/cpu/intel: Fix the constant_tsc model check for Pentium 4s

Sohil Mehta
2025-07-30T14:42:42Z

Hi Suchit,

The patch looks good to me except for a few nits below.

On 7/29/2025 9:26 PM, 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.

A blank line here would be useful to separate the two paragraphs.

> The error was introduced while replacing the x86_model check with a VFM
> one. The original check was as follows:

Maybe to make it more precise and avoid confusion.

The original check before the erroneous code 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.
>
> Fixes: fadb6f569b10 ("x86/cpu/intel: Limit the non-architectural constant_tsc model checks")
>
> Cc: <stable@vger.kernel.org> # v6.15
>
> Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>
>

Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
(without the blank lines as Greg mentioned)

>
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 076eaa41b8c8..6f5bd5dbc249 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) ||

The alignment here seems to have changed because the extra space after
INTEL_P4_PRESCOTT was moved before it. The current code has alignment
matched with the below line. It isn't a big deal, but keeps the code
easier to read.

> (c->x86_vfm >= INTEL_CORE_YONAH && c->x86_vfm <= INTEL_IVYBRIDGE)) {
> set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
> }