linux-kernel - Re: [PATCH v2] x86/intel: Fix always false range check in x86

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Message-ID: <CAO9wTFgB_5tiVj5boPpgWtYd2O+49oyUemdcWVP9+vpZri=Nmw@mail.gmail.com>
Date: Wed, 30 Jul 2025 10:00:11 +0530
From: Suchit Karunakaran <suchitkarunakaran@...il.com>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, hpa@...or.com, darwi@...utronix.de, 
	peterz@...radead.org, ravi.bangoria@....com, skhan@...uxfoundation.org, 
	linux-kernel-mentees@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/intel: Fix always false range check in x86_vfm
 model matching

On Wed, 30 Jul 2025 at 00:20, Sohil Mehta <sohil.mehta@...el.com> wrote:
>
> Though the title is correct, it is better to make it more precise.
> Something like:
>
> x86/cpu/intel: Fix the constant_tsc model check for Pentium 4s
>
> On 7/28/2025 9:26 PM, Suchit Karunakaran wrote:
> > Fix a logic bug in early_init_intel() where a conditional range check:
> > (c->x86_vfm >= INTEL_P4_PRESCOTT && c->x86_vfm <= INTEL_P4_WILLAMETTE)
> > was always false due to (PRESCOTT) being numerically greater than the
> > upper bound (WILLAMETTE). This triggers:-Werror=logical-op:
> > logical ‘and’ of mutually exclusive tests is always false
>
> We can focus on the why instead of how this error was found. How about
> wording it as below?
>
> 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.
>
> > The fix corrects the constant ordering to ensure the range is valid:
> > (c->x86_vfm >=  INTEL_P4_PRESCOTT && c->x86_vfm <= INTEL_P4_CEDARMILL)
> >
>
> This line is unnecessary. Changes that are easily observable in the diff
> below don't need to be described in the commit message.
>
> > Fixes: fadb6f569b10 ("x86/cpu/intel: Limit the non-architectural
> > constant_tsc model checks")
> >
>
> The "Fixes:" line doesn't need to be broken down into multiple lines.
> All of it can be on the same line.
>
> For patches being reviewed for upstream, adding a stable Cc (Option 1 in
> the stable documentation) is the easiest and preferred way to
> automatically trigger submissions for stable.
>
> In this case, include the below Cc: line after the Fixes: line.
>
> Cc: <stable@...r.kernel.org> # v6.15
>
> > Signed-off-by: Suchit Karunakaran <suchitkarunakaran@...il.com>
> >
> > Changes since v1:
> > - Fix incorrect logic
>
> Patch-to-patch changes should be below the --- line. That way they will
> get ignored when the patch is applied to the maintainer's tree.
>
> > ---
> >  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..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) ||
> >                  (c->x86_vfm >= INTEL_CORE_YONAH  && c->x86_vfm <= INTEL_IVYBRIDGE)) {
> >               set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
> >       }
>

Hi Sohil,
Thank you very much for your valuable feedback. I have incorporated
all the suggested changes and sent out the v3 patch. Could you please
review it?

Thanks,
Suchit