Re: [PATCH v2 15/17] x86/cpu/intel: Bound the non-architectural constant_tsc model checks

[Dave Hansen <dave.hansen@intel.com>]

On 2/11/25 11:44, Sohil Mehta wrote:
> Constant TSC has been architectural on Intel CPUs for a while. Supported
> CPUs use the architectural Invariant TSC bit in CPUID.80000007. A
> Family-model check is not required for these CPUs.
> 
> Prevent unnecessary confusion but restricting the model specific checks
> to CPUs that need it and moving it closer to the architectural check.
> 
> Invariant TSC was likely introduced around the Nehalam timeframe on the
> Xeon side and Saltwell timeframe on the Atom side.  Due to interspersed
> model numbers extend the non-architectural capability setting until
> Ivybridge to be safe.

How about:

X86_FEATURE_CONSTANT_TSC is a Linux-defined, synthesized feature flag.
It is used across several vendors. Intel CPUs will set the feature when
the architectural CPUID.80000007.EDX[1] bit is set. There are also some
Intel CPUs that have the X86_FEATURE_CONSTANT_TSC behavior but don't
enumerate it with the architectural bit.  Those currently have a model
range check.

Today, virtually all of the CPUs that have the CPUID bit *also* match
the "model >= 0x0e" check. This is confusing. Instead of an open-ended
check, pick some models (INTEL_IVYBRIDGE and P4_WILLAMETTE) as the end
of goofy CPUs that should enumerate the bit but don't.  These models are
relatively arbitrary but conservative pick for this.

This makes it obvious that later CPUs (like family 18+) no longer need
to synthesize X86_FEATURE_CONSTANT_TSC.


> +	/* Some older CPUs have invariant TSC but may not report it architecturally via 8000_0007 */
> +	if ((c->x86_vfm >= INTEL_P4_PRESCOTT && c->x86_vfm <= INTEL_P4_WILLAMETTE) ||
> +	    (c->x86_vfm >= INTEL_CORE_YONAH && c->x86_vfm <= INTEL_IVYBRIDGE))
> +		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);

Please do vertically align this too.

Would it make logical sense to do:

        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 ...

?

That would make it *totally* clear that it's an either/or situation.  Right?