[PATCH] powerpc/perf: Fix PMU callbacks to clear pending PMI before resetting an overflown PMC

[Athira Rajeev <atrajeev@linux.vnet.ibm.com>]

Running perf fuzzer showed below in dmesg logs:
"Can't find PMC that caused IRQ"

This means a PMU exception happened, but none of the PMC's (Performance
Monitor Counter) were found to be overflown. There are some corner cases
that clears the PMCs after PMI gets masked. In such cases, the perf
interrupt handler will not find the active PMC values that had caused
the overflow and thus leads to this message while replaying.

Case 1: PMU Interrupt happens during replay of other interrupts and
counter values gets cleared by PMU callbacks before replay:

During replay of interrupts like timer, __do_irq and doorbell exception, we
conditionally enable interrupts via may_hard_irq_enable(). This could
potentially create a window to generate a PMI. Since irq soft mask is set
to ALL_DISABLED, the PMI will get masked here. We could get IPIs run before
perf interrupt is replayed and the PMU events could deleted or stopped.
This will change the PMU SPR values and resets the counters. Snippet of
ftrace log showing PMU callbacks invoked in "__do_irq":

<idle>-0 [051] dns. 132025441306354: __do_irq <-call_do_irq
<idle>-0 [051] dns. 132025441306430: irq_enter <-__do_irq
<idle>-0 [051] dns. 132025441306503: irq_enter_rcu <-__do_irq
<idle>-0 [051] dnH. 132025441306599: xive_get_irq <-__do_irq
<<>>
<idle>-0 [051] dnH. 132025441307770: generic_smp_call_function_single_interrupt <-smp_ipi_demux_relaxed
<idle>-0 [051] dnH. 132025441307839: flush_smp_call_function_queue <-smp_ipi_demux_relaxed
<idle>-0 [051] dnH. 132025441308057: _raw_spin_lock <-event_function
<idle>-0 [051] dnH. 132025441308206: power_pmu_disable <-perf_pmu_disable
<idle>-0 [051] dnH. 132025441308337: power_pmu_del <-event_sched_out
<idle>-0 [051] dnH. 132025441308407: power_pmu_read <-power_pmu_del
<idle>-0 [051] dnH. 132025441308477: read_pmc <-power_pmu_read
<idle>-0 [051] dnH. 132025441308590: isa207_disable_pmc <-power_pmu_del
<idle>-0 [051] dnH. 132025441308663: write_pmc <-power_pmu_del
<idle>-0 [051] dnH. 132025441308787: power_pmu_event_idx <-perf_event_update_userpage
<idle>-0 [051] dnH. 132025441308859: rcu_read_unlock_strict <-perf_event_update_userpage
<idle>-0 [051] dnH. 132025441308975: power_pmu_enable <-perf_pmu_enable
<<>>
<idle>-0 [051] dnH. 132025441311108: irq_exit <-__do_irq
<idle>-0 [051] dns. 132025441311319: performance_monitor_exception <-replay_soft_interrupts

Case 2: PMI's masked during local_* operations, example local_add.
If the local_add operation happens within a local_irq_save, replay of
PMI will be during local_irq_restore. Similar to case 1, this could
also create a window before replay where PMU events gets deleted or
stopped.

Patch adds a fix to update the PMU callback functions (del,stop,enable) to
check for pending perf interrupt. If there is an overflown PMC and pending
perf interrupt indicated in Paca, clear the PMI bit in paca to drop that
sample. In case of power_pmu_del, also clear the MMCR0 PMAO bit which
otherwise could lead to spurious interrupts in some corner cases. Example,
a timer after power_pmu_del which will re-enable interrupts since PMI is
cleared and triggers a PMI again since PMAO bit is still set.

We can't just replay PMI any time. Hence this approach is preferred rather
than replaying PMI before resetting overflown PMC. Patch also documents
core-book3s on a race condition which can trigger these PMC messages during
idle path in PowerNV.

Fixes: f442d004806e ("powerpc/64s: Add support to mask perf interrupts and replay them")
Reported-by: Nageswara R Sastry <nasastry@in.ibm.com>
Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Suggested-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/pmc.h  | 11 +++++++++
 arch/powerpc/perf/core-book3s.c | 55 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/arch/powerpc/include/asm/pmc.h b/arch/powerpc/include/asm/pmc.h
index c6bbe9778d3c..97b4bd8de25b 100644
--- a/arch/powerpc/include/asm/pmc.h
+++ b/arch/powerpc/include/asm/pmc.h
@@ -34,11 +34,22 @@ static inline void ppc_set_pmu_inuse(int inuse)
 #endif
 }
 
+static inline int clear_paca_irq_pmi(void)
+{
+	if (get_paca()->irq_happened & PACA_IRQ_PMI) {
+		WARN_ON_ONCE(mfmsr() & MSR_EE);
+		get_paca()->irq_happened &= ~PACA_IRQ_PMI;
+		return 1;
+	}
+	return 0;
+}
+
 extern void power4_enable_pmcs(void);
 
 #else /* CONFIG_PPC64 */
 
 static inline void ppc_set_pmu_inuse(int inuse) { }
+static inline int clear_paca_irq_pmi(void) { return 0; }
 
 #endif
 
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 766f064f00fb..18ca3c90f866 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -847,6 +847,20 @@ static void write_pmc(int idx, unsigned long val)
 	}
 }
 
+static int pmc_overflown(int idx)
+{
+	unsigned long val[8];
+	int i;
+
+	for (i = 0; i < ppmu->n_counter; i++)
+		val[i] = read_pmc(i + 1);
+
+	if ((int)val[idx-1] < 0)
+		return 1;
+
+	return 0;
+}
+
 /* Called from sysrq_handle_showregs() */
 void perf_event_print_debug(void)
 {
@@ -1438,6 +1452,15 @@ static void power_pmu_enable(struct pmu *pmu)
 		event = cpuhw->event[i];
 		if (event->hw.idx && event->hw.idx != hwc_index[i] + 1) {
 			power_pmu_read(event);
+			/*
+			 * if the PMC corresponding to event->hw.idx is
+			 * overflown, check if there is any pending perf
+			 * interrupt set in paca. If so, disable the interrupt
+			 * by clearing the paca bit for PMI since we are going
+			 * to reset the PMC.
+			 */
+			if (pmc_overflown(event->hw.idx))
+				clear_paca_irq_pmi();
 			write_pmc(event->hw.idx, 0);
 			event->hw.idx = 0;
 		}
@@ -1474,6 +1497,10 @@ static void power_pmu_enable(struct pmu *pmu)
 		event->hw.idx = idx;
 		if (event->hw.state & PERF_HES_STOPPED)
 			val = 0;
+
+		/* See above for clear_paca_irq_pmi */
+		if (pmc_overflown(event->hw.idx))
+			clear_paca_irq_pmi();
 		write_pmc(idx, val);
 
 		perf_event_update_userpage(event);
@@ -1619,6 +1646,7 @@ static void power_pmu_del(struct perf_event *event, int ef_flags)
 	struct cpu_hw_events *cpuhw;
 	long i;
 	unsigned long flags;
+	unsigned long val_mmcr0;
 
 	local_irq_save(flags);
 	perf_pmu_disable(event->pmu);
@@ -1636,6 +1664,22 @@ static void power_pmu_del(struct perf_event *event, int ef_flags)
 			--cpuhw->n_events;
 			ppmu->disable_pmc(event->hw.idx - 1, &cpuhw->mmcr);
 			if (event->hw.idx) {
+				/*
+				 * if the PMC corresponding to event->hw.idx is
+				 * overflown, check if there is any pending perf
+				 * interrupt set in paca. If so, disable the interrupt
+				 * and clear the MMCR0 PMAO bit since we are going
+				 * to reset the PMC and delete the event.
+				 */
+				if (pmc_overflown(event->hw.idx)) {
+					if (clear_paca_irq_pmi()) {
+						val_mmcr0 = mfspr(SPRN_MMCR0);
+						val_mmcr0 &= ~MMCR0_PMAO;
+						write_mmcr0(cpuhw, val_mmcr0);
+						mb();
+						isync();
+					}
+				}
 				write_pmc(event->hw.idx, 0);
 				event->hw.idx = 0;
 			}
@@ -1714,6 +1758,8 @@ static void power_pmu_stop(struct perf_event *event, int ef_flags)
 
 	local_irq_save(flags);
 	perf_pmu_disable(event->pmu);
+	if (pmc_overflown(event->hw.idx))
+		clear_paca_irq_pmi();
 
 	power_pmu_read(event);
 	event->hw.state |= PERF_HES_STOPPED | PERF_HES_UPTODATE;
@@ -2343,6 +2389,15 @@ static void __perf_event_interrupt(struct pt_regs *regs)
 			}
 		}
 	}
+
+	/*
+	 * During system wide profling or while specific CPU
+	 * is monitored for an event, some corner cases could
+	 * cause PMC to overflow in idle path. This will trigger
+	 * a PMI after waking up from idle. Since counter values
+	 * are _not_ saved/restored in idle path, can lead to
+	 * below "Can't find PMC" message.
+	 */
 	if (unlikely(!found) && !arch_irq_disabled_regs(regs))
 		printk_ratelimited(KERN_WARNING "Can't find PMC that caused IRQ\n");
 
-- 
1.8.3.1