perf/x86: Add helper to obtain performance counter index

perf_event_read_local() is the safest way to obtain measurements
associated with performance events. In some cases the overhead
introduced by perf_event_read_local() affects the measurements and the
use of rdpmcl() is needed. rdpmcl() requires the index
of the performance counter used so a helper is introduced to determine
the index used by a provided performance event.

The index used by a performance event may change when interrupts are
enabled. A check is added to ensure that the index is only accessed
with interrupts disabled. Even with this check the use of this counter
needs to be done with care to ensure it is queried and used within the
same disabled interrupts section.

This change introduces a new checkpatch warning:
CHECK: extern prototypes should be avoided in .h files
+extern int x86_perf_rdpmc_index(struct perf_event *event);

This warning was discussed and designated as a false positive in
http://lkml.kernel.org/r/20180919091759.GZ24124@hirez.programming.kicks-ass.net

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Cc: acme@kernel.org
Cc: gavin.hindman@intel.com
Cc: jithu.joseph@intel.com
Cc: dave.hansen@intel.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/b277ffa78a51254f5414f7b1bc1923826874566e.1537377064.git.reinette.chatre@intel.com

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
index 12f54082f4c8..b2cf84c35a6d 100644
--- a/arch/x86/include/asm/perf_event.h
+++ b/arch/x86/include/asm/perf_event.h
@@ -270,6 +270,7 @@ struct perf_guest_switch_msr {
 extern struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr);
 extern void perf_get_x86_pmu_capability(struct x86_pmu_capability *cap);
 extern void perf_check_microcode(void);
+extern int x86_perf_rdpmc_index(struct perf_event *event);
 #else
 static inline struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
 {