[PATCH v4 6/7] perf/x86/amd/core: Add PerfMonV2 overflow handling

[Sandipan Das]

If AMD Performance Monitoring Version 2 (PerfMonV2) is
supported, use a new scheme to process Core PMC overflows
in the NMI handler using the new global control and status
registers. This will be bypassed on unsupported hardware
(x86_pmu.version < 2).

In x86_pmu_handle_irq(), overflows are detected by testing
the contents of the PERF_CTR register for each active PMC in
a loop. The new scheme instead inspects the overflow bits of
the global status register.

The Performance Counter Global Status (PerfCntrGlobalStatus)
register has overflow (PerfCntrOvfl) bits for each PMC. This
is, however, a read-only MSR. To acknowledge that overflows
have been processed, the NMI handler must clear the bits by
writing to the PerfCntrGlobalStatusClr register.

In x86_pmu_handle_irq(), PMCs counting the same event that
are started and stopped at the same time record slightly
different counts due to delays in between reads from the
PERF_CTR registers. This is fixed by stopping and starting
the PMCs at the same before and with a single write to the
Performance Counter Global Control (PerfCntrGlobalCtl) upon
entering and before exiting the NMI handler.

Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
---
 arch/x86/events/amd/core.c | 144 ++++++++++++++++++++++++++++++++++---
 1 file changed, 133 insertions(+), 11 deletions(-)

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 5b100a5f8489..5de2c833acf2 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -8,6 +8,7 @@
 #include <linux/delay.h>
 #include <linux/jiffies.h>
 #include <asm/apicdef.h>
+#include <asm/apic.h>
 #include <asm/nmi.h>
 
 #include "../perf_event.h"
@@ -669,6 +670,45 @@ static inline void amd_pmu_set_global_ctl(u64 ctl)
 	wrmsrl(MSR_AMD64_PERF_CNTR_GLOBAL_CTL, ctl);
 }
 
+static inline u64 amd_pmu_get_global_status(void)
+{
+	u64 status;
+
+	/* PerfCntrGlobalStatus is read-only */
+	rdmsrl(MSR_AMD64_PERF_CNTR_GLOBAL_STATUS, status);
+
+	return status & amd_pmu_global_cntr_mask;
+}
+
+static inline void amd_pmu_ack_global_status(u64 status)
+{
+	/*
+	 * PerfCntrGlobalStatus is read-only but an overflow acknowledgment
+	 * mechanism exists; writing 1 to a bit in PerfCntrGlobalStatusClr
+	 * clears the same bit in PerfCntrGlobalStatus
+	 */
+
+	/* Only allow modifications to PerfCntrGlobalStatus.PerfCntrOvfl */
+	status &= amd_pmu_global_cntr_mask;
+	wrmsrl(MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR, status);
+}
+
+static bool amd_pmu_test_overflow_topbit(int idx)
+{
+	u64 counter;
+
+	rdmsrl(x86_pmu_event_addr(idx), counter);
+
+	return !(counter & BIT_ULL(x86_pmu.cntval_bits - 1));
+}
+
+static bool amd_pmu_test_overflow_status(int idx)
+{
+	return amd_pmu_get_global_status() & BIT_ULL(idx);
+}
+
+DEFINE_STATIC_CALL(amd_pmu_test_overflow, amd_pmu_test_overflow_topbit);
+
 /*
  * When a PMC counter overflows, an NMI is used to process the event and
  * reset the counter. NMI latency can result in the counter being updated
@@ -681,7 +721,6 @@ static inline void amd_pmu_set_global_ctl(u64 ctl)
 static void amd_pmu_wait_on_overflow(int idx)
 {
 	unsigned int i;
-	u64 counter;
 
 	/*
 	 * Wait for the counter to be reset if it has overflowed. This loop
@@ -689,8 +728,7 @@ static void amd_pmu_wait_on_overflow(int idx)
 	 * forever...
 	 */
 	for (i = 0; i < OVERFLOW_WAIT_COUNT; i++) {
-		rdmsrl(x86_pmu_event_addr(idx), counter);
-		if (counter & (1ULL << (x86_pmu.cntval_bits - 1)))
+		if (!static_call(amd_pmu_test_overflow)(idx))
 			break;
 
 		/* Might be in IRQ context, so can't sleep */
@@ -830,6 +868,24 @@ static void amd_pmu_del_event(struct perf_event *event)
  * handled a counter. When an un-handled NMI is received, it will be claimed
  * only if arriving within that window.
  */
+static inline int amd_pmu_adjust_nmi_window(int handled)
+{
+	/*
+	 * If a counter was handled, record a timestamp such that un-handled
+	 * NMIs will be claimed if arriving within that window.
+	 */
+	if (handled) {
+		this_cpu_write(perf_nmi_tstamp, jiffies + perf_nmi_window);
+
+		return handled;
+	}
+
+	if (time_after(jiffies, this_cpu_read(perf_nmi_tstamp)))
+		return NMI_DONE;
+
+	return NMI_HANDLED;
+}
+
 static int amd_pmu_handle_irq(struct pt_regs *regs)
 {
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
@@ -857,20 +913,84 @@ static int amd_pmu_handle_irq(struct pt_regs *regs)
 	if (pmu_enabled)
 		amd_pmu_enable_all(0);
 
+	return amd_pmu_adjust_nmi_window(handled);
+}
+
+static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
+{
+	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+	struct perf_sample_data data;
+	struct hw_perf_event *hwc;
+	struct perf_event *event;
+	int handled = 0, idx;
+	u64 status, mask;
+	bool pmu_enabled;
+
 	/*
-	 * If a counter was handled, record a timestamp such that un-handled
-	 * NMIs will be claimed if arriving within that window.
+	 * Save the PMU state as it needs to be restored when leaving the
+	 * handler
 	 */
-	if (handled) {
-		this_cpu_write(perf_nmi_tstamp, jiffies + perf_nmi_window);
+	pmu_enabled = cpuc->enabled;
+	cpuc->enabled = 0;
 
-		return handled;
+	/* Stop counting */
+	amd_pmu_v2_disable_all();
+
+	status = amd_pmu_get_global_status();
+
+	/* Check if any overflows are pending */
+	if (!status)
+		goto done;
+
+	for (idx = 0; idx < x86_pmu.num_counters; idx++) {
+		if (!test_bit(idx, cpuc->active_mask))
+			continue;
+
+		event = cpuc->events[idx];
+		hwc = &event->hw;
+		x86_perf_event_update(event);
+		mask = BIT_ULL(idx);
+
+		if (!(status & mask))
+			continue;
+
+		/* Event overflow */
+		handled++;
+		perf_sample_data_init(&data, 0, hwc->last_period);
+
+		if (!x86_perf_event_set_period(event))
+			continue;
+
+		if (perf_event_overflow(event, &data, regs))
+			x86_pmu_stop(event, 0);
+
+		status &= ~mask;
 	}
 
-	if (time_after(jiffies, this_cpu_read(perf_nmi_tstamp)))
-		return NMI_DONE;
+	/*
+	 * It should never be the case that some overflows are not handled as
+	 * the corresponding PMCs are expected to be inactive according to the
+	 * active_mask
+	 */
+	WARN_ON(status > 0);
 
-	return NMI_HANDLED;
+	/* Clear overflow bits */
+	amd_pmu_ack_global_status(~status);
+
+	/*
+	 * Unmasking the LVTPC is not required as the Mask (M) bit of the LVT
+	 * PMI entry is not set by the local APIC when a PMC overflow occurs
+	 */
+	inc_irq_stat(apic_perf_irqs);
+
+done:
+	cpuc->enabled = pmu_enabled;
+
+	/* Resume counting only if PMU is active */
+	if (pmu_enabled)
+		amd_pmu_v2_enable_all(0);
+
+	return amd_pmu_adjust_nmi_window(handled);
 }
 
 static struct event_constraint *
@@ -1256,6 +1376,8 @@ static int __init amd_core_pmu_init(void)
 		x86_pmu.enable_all = amd_pmu_v2_enable_all;
 		x86_pmu.disable_all = amd_pmu_v2_disable_all;
 		x86_pmu.enable = amd_pmu_v2_enable_event;
+		x86_pmu.handle_irq = amd_pmu_v2_handle_irq;
+		static_call_update(amd_pmu_test_overflow, amd_pmu_test_overflow_status);
 	}
 
 	/*
-- 
2.34.1