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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: sohilmeh

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 */