x86/cpu/intel: Limit the non-architectural constant_tsc model checks

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.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20250219184133.816753-14-sohil.mehta@intel.com

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 2181304dc7d3ef964188b9c9be856c46051cbd7a..4cbb2e69bea1974e049a203ee135ac14a30943d1 100644 (file)
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -201,10 +201,6 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 {
        u64 misc_enable;
 
-       if ((c->x86 == 0xf && c->x86_model >= 0x03) ||
-               (c->x86 == 0x6 && c->x86_model >= 0x0e))
-               set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
-
        if (c->x86 >= 6 && !cpu_has(c, X86_FEATURE_IA64))
                c->microcode = intel_get_microcode_revision();
 
@@ -257,10 +253,16 @@ static void early_init_intel(struct cpuinfo_x86 *c)
         *
         * It is also reliable across cores and sockets. (but not across
         * cabinets - we turn it off in that case explicitly.)
+        *
+        * Use a model-specific check for some older CPUs that have invariant
+        * TSC but may not report it architecturally via 8000_0007.
         */
        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) ||
+                  (c->x86_vfm >= INTEL_CORE_YONAH  && c->x86_vfm <= INTEL_IVYBRIDGE)) {
+               set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
        }
 
        /* Penwell and Cloverview have the TSC which doesn't sleep on S3 */