perf/x86: Refuse to export capabilities for hybrid PMUs

Now that KVM disables vPMU support on hybrid CPUs, WARN and return zeros
if perf_get_x86_pmu_capability() is invoked on a hybrid CPU.  The helper
doesn't provide an accurate accounting of the PMU capabilities for hybrid
CPUs and needs to be enhanced if KVM, or anything else outside of perf,
wants to act on the PMU capabilities.

Cc: [email protected]
Cc: Andrew Cooper <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Andi Kleen <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: sean-jc

arch/x86/events/core.c

@@ -2974,17 +2974,19 @@ unsigned long perf_misc_flags(struct pt_regs *regs)
 
 void perf_get_x86_pmu_capability(struct x86_pmu_capability *cap)
 {
-	if (!x86_pmu_initialized()) {
+	/* This API doesn't currently support enumerating hybrid PMUs. */
+	if (WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_HYBRID_CPU)) ||
+	    !x86_pmu_initialized()) {
 		memset(cap, 0, sizeof(*cap));
 		return;
 	}
 
-	cap->version		= x86_pmu.version;
 	/*
-	 * KVM doesn't support the hybrid PMU yet.
-	 * Return the common value in global x86_pmu,
-	 * which available for all cores.
+	 * Note, hybrid CPU models get tracked as having hybrid PMUs even when
+	 * all E-cores are disabled via BIOS.  When E-cores are disabled, the
+	 * base PMU holds the correct number of counters for P-cores.
 	 */
+	cap->version		= x86_pmu.version;
 	cap->num_counters_gp	= x86_pmu.num_counters;
 	cap->num_counters_fixed	= x86_pmu.num_counters_fixed;
 	cap->bit_width_gp	= x86_pmu.cntval_bits;