[PATCH v2 2/2] perf/x86: Refuse to export capabilities for hybrid PMUs

[Sean Christopherson]

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: stable@xxxxxxxxxxxxxxx
Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20220818181530.2355034-1-kan.liang@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/events/core.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 85a63a41c471..d096b04bf80e 100644
--- a/arch/x86/events/core.c
+++ b/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;
 	}
 
+	/*
+	 * 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;
-	/*
-	 * KVM doesn't support the hybrid PMU yet.
-	 * Return the common value in global x86_pmu,
-	 * which available for all cores.
-	 */
 	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;
-- 
2.39.1.519.gcb327c4b5f-goog