[RFC PATCH v2] perf: honouring the cpuid for number of fixed counters in hypervisors

[Imre Palik <imrep.amz@gmail.com>]

From: Imre Palik <imrep@amazon.de>

perf doesn't seem to honour the number of fixed counters specified by cpuid
leaf 0xa.  It always assume that Intel CPUs have at least 3 fixed counters.

So if some of the fixed counters are masked out by the hypervisor, it still
tries to check/set them.

This patch makes perf behave nicer when the kernel is running under a
hypervisor that doesn't expose all the counters.

This patch contains some ideas from Matt Wilson.

Signed-off-by: Imre Palik <imrep@amazon.de>
Cc: Matt Wilson <msw@amazon.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/events/intel/core.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index a3a9eb8..e06d071 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3607,10 +3607,14 @@ __init int intel_pmu_init(void)
 
 	/*
 	 * Quirk: v2 perfmon does not report fixed-purpose events, so
-	 * assume at least 3 events:
+	 * assume at least 3 events, when not running in a hypervisor:
 	 */
-	if (version > 1)
-		x86_pmu.num_counters_fixed = max((int)edx.split.num_counters_fixed, 3);
+	if (version > 1) {
+		if (static_cpu_has(X86_FEATURE_HYPERVISOR))
+			x86_pmu.num_counters_fixed = edx.split.num_counters_fixed;
+		else
+			x86_pmu.num_counters_fixed = max((int)edx.split.num_counters_fixed, 3);
+	}
 
 	if (boot_cpu_has(X86_FEATURE_PDCM)) {
 		u64 capabilities;
-- 
2.10.1