perf/x86/amd/lbr: Detect LbrExtV2 support

AMD Last Branch Record Extension Version 2 (LbrExtV2) is driven by Core PMC
overflows. It records recently taken branches up to the moment when the PMC
overflow occurs.

Detect the feature during PMU initialization and set the branch stack depth
using CPUID leaf 0x80000022 EBX.
Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/fc6e45378ada258f1bab79b0de6e05c393a8f1dd.1660211399.git.sandipan.das@amd.com

arch/x86/events/amd/Makefile

 # SPDX-License-Identifier: GPL-2.0
-obj-$(CONFIG_CPU_SUP_AMD)		+= core.o
+obj-$(CONFIG_CPU_SUP_AMD)		+= core.o lbr.o
 obj-$(CONFIG_PERF_EVENTS_AMD_BRS)	+= brs.o
 obj-$(CONFIG_PERF_EVENTS_AMD_POWER)	+= power.o
 obj-$(CONFIG_X86_LOCAL_APIC)		+= ibs.o

arch/x86/events/amd/core.c

@@ -1374,10 +1374,11 @@ static int __init amd_core_pmu_init(void)
 		x86_pmu.flags |= PMU_FL_PAIR;
 	}
 
-	/*
-	 * BRS requires special event constraints and flushing on ctxsw.
-	 */
-	if (boot_cpu_data.x86 >= 0x19 && !amd_brs_init()) {
+	/* LBR and BRS are mutually exclusive features */
+	if (amd_pmu_lbr_init() && !amd_brs_init()) {
+		/*
+		 * BRS requires special event constraints and flushing on ctxsw.
+		 */
 		x86_pmu.get_event_constraints = amd_get_event_constraints_f19h;
 		x86_pmu.sched_task = amd_pmu_brs_sched_task;
 		x86_pmu.limit_period = amd_pmu_limit_period;

arch/x86/events/amd/lbr.c

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/perf_event.h>
+#include <asm/perf_event.h>
+
+#include "../perf_event.h"
+
+__init int amd_pmu_lbr_init(void)
+{
+	union cpuid_0x80000022_ebx ebx;
+
+	if (x86_pmu.version < 2 || !boot_cpu_has(X86_FEATURE_AMD_LBR_V2))
+		return -EOPNOTSUPP;
+
+	/* Set number of entries */
+	ebx.full = cpuid_ebx(EXT_PERFMON_DEBUG_FEATURES);
+	x86_pmu.lbr_nr = ebx.split.lbr_v2_stack_sz;
+
+	pr_cont("%d-deep LBR, ", x86_pmu.lbr_nr);
+
+	return 0;
+}

arch/x86/events/perf_event.h

@@ -1232,6 +1232,8 @@ static inline bool fixed_counter_disabled(int i, struct pmu *pmu)
 
 int amd_pmu_init(void);
 
+int amd_pmu_lbr_init(void);
+
 #ifdef CONFIG_PERF_EVENTS_AMD_BRS
 
 #define AMD_FAM19H_BRS_EVENT 0xc4 /* RETIRED_TAKEN_BRANCH_INSTRUCTIONS */

arch/x86/include/asm/perf_event.h

@@ -207,7 +207,8 @@ union cpuid_0x80000022_ebx {
 	struct {
 		/* Number of Core Performance Counters */
 		unsigned int	num_core_pmc:4;
-		unsigned int	reserved:6;
+		/* Number of available LBR Stack Entries */
+		unsigned int	lbr_v2_stack_sz:6;
 		/* Number of Data Fabric Counters */
 		unsigned int	num_df_pmc:6;
 	} split;