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

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Message-ID: <CAO9wTFiQLLTFcTew+ymC85m8dNLu_KcJOYj9vu2ke0fcUYW-Cw@mail.gmail.com>
Date: Tue, 29 Jul 2025 09:47:00 +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] x86/intel: Fix always false range check in x86_vfm model matching

On Tue, 29 Jul 2025 at 00:26, Sohil Mehta <sohil.mehta@...el.com> wrote:
>
> On 7/28/2025 11:09 AM, 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
> > The fix corrects the constant ordering to ensure the range is valid:
> >     (c->x86_vfm >= INTEL_P4_WILLAMETTE && c->x86_vfm <= INTEL_P4_PRESCOTT)
> >
>
> Thank you for reporting this issue!
>
> This is indeed a bug, but the resolution proposed here seems incorrect.
>
> > Signed-off-by: Suchit Karunakaran <suchitkarunakaran@...il.com>
> > ---
> >  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..3ec1555cbdc3 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_WILLAMETTE && c->x86_vfm <= INTEL_P4_PRESCOTT) ||
> >                  (c->x86_vfm >= INTEL_CORE_YONAH  && c->x86_vfm <= INTEL_IVYBRIDGE)) {
> >               set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
> >       }
>
> See commit fadb6f569b10 ("x86/cpu/intel: Limit the non-architectural
> constant_tsc model checks")
>
> The original check that this replaced is:
>
>         if ((c->x86 == 0xf && c->x86_model >= 0x03) ||
>                 (c->x86 == 0x6 && c->x86_model >= 0x0e))
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>
> The first check is supposed to cover all P4 models starting with model 3
> (INTEL_P4_PRESCOTT). In order to set an upper bound for Family 15, the
> check should cover all models until model 6 (INTEL_P4_CEDARMILL) which
> is the last model released in that family.
>
> The error that I made was to use an incorrect upper bound. The actual
> fix should be this:
>
> s/INTEL_P4_WILLAMETTE/INTEL_P4_CEDARMILL
>
> > @@ -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);
> >         }
>
> Also, please add a
> Fixes: fadb6f569b10 ("x86/cpu/intel: Limit the non-architectural
> constant_tsc model checks")
>

Thanks for reviewing. I'll send a v2 patch with the suggested changes.