Subject: [PATCH V4 1/1] powerpc/perf: Fix PMU callbacks to clear pending PMI before resetting an overflown PMC

From: Athira Rajeev <[email-masked]>
Date: Wed, 21 Jul 2021 01:48:29 -0400

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 function 'power_pmu_disable' 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. Clearing of PMI bit is done in 'power_pmu_disable' since disable is
invoked before any event gets deleted/stopped. With this fix, if there are
more than one event running in the PMU, there is a chance that we clear the
PMI bit for the event which is not getting deleted/stopped. The other
events may still remain active. Hence to make sure we don't drop valid
sample in such cases, another check is added in power_pmu_enable. This
checks if there is an overflown PMC found among the active events and if
so enable back the PMI bit. Two new helper functions are introduced to
clear/set the PMI, ie 'clear_pmi_irq_pending' and 'set_pmi_irq_pending'.
Helper function 'pmi_irq_pending' is introduced to give a warning if
there is pending PMI bit in paca, but no PMC is overflown.

Also there are corner cases which results in performance monitor interrupts
getting triggered during power_pmu_disable. This happens since PMXE bit is
not cleared along with disabling of other MMCR0 bits in the pmu_disable.
Such PMI's could leave the PMU running and could trigger PMI again which
will set MMCR0 PMAO bit. This could lead to spurious interrupts in some
corner cases. Example, a timer after power_pmu_del which will re-enable
interrupts and triggers a PMI again since PMAO bit is still set. But fails
to find valid overflow since PMC get cleared in power_pmu_del. Patch
fixes this by disabling PMXE along with disabling of other MMCR0 bits
in power_pmu_disable.

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 <[email-masked]>
Suggested-by: Nicholas Piggin <[email-masked]>
Suggested-by: Madhavan Srinivasan <[email-masked]>
Signed-off-by: Athira Rajeev <[email-masked]>
Reviewed-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/include/asm/hw_irq.h | 38 +++++++++++++++++++++++++
 arch/powerpc/perf/core-book3s.c   | 59 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 96 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
index 21cc571..d4e2d74 100644
--- a/arch/powerpc/include/asm/hw_irq.h
+++ b/arch/powerpc/include/asm/hw_irq.h
@@ -224,6 +224,40 @@ static inline bool arch_irqs_disabled(void)
 	return arch_irqs_disabled_flags(arch_local_save_flags());
 }
 
+static inline void set_pmi_irq_pending(void)
+{
+	/*
+	 * Invoked from PMU callback functions to set
+	 * PMI bit in Paca. This has to be called with
+	 * irq's disabled ( via hard_irq_disable ).
+	 */
+	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
+		WARN_ON_ONCE(mfmsr() & MSR_EE);
+	get_paca()->irq_happened |= PACA_IRQ_PMI;
+}
+
+static inline void clear_pmi_irq_pending(void)
+{
+	/*
+	 * Invoked from PMU callback functions to clear
+	 * the pending PMI bit in Paca.
+	 */
+	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
+		WARN_ON_ONCE(mfmsr() & MSR_EE);
+	get_paca()->irq_happened &= ~PACA_IRQ_PMI;
+}
+
+static inline int pmi_irq_pending(void)
+{
+	/*
+	 * Invoked from PMU callback functions to check
+	 * if there is a pending PMI bit in Paca.
+	 */
+	if (get_paca()->irq_happened & PACA_IRQ_PMI)
+		return 1;
+	return 0;
+}
+
 #ifdef CONFIG_PPC_BOOK3S
 /*
  * To support disabling and enabling of irq with PMI, set of
@@ -408,6 +442,10 @@ static inline void do_hard_irq_enable(void)
 	BUILD_BUG();
 }
 
+static inline void clear_pmi_irq_pending(void) { }
+static inline void set_pmi_irq_pending(void) { }
+static inline int pmi_irq_pending(void) { return 0; }
+
 static inline void irq_soft_mask_regs_set_state(struct pt_regs *regs, unsigned long val)
 {
 }
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index bb0ee71..ad29220 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -848,6 +848,19 @@ static void write_pmc(int idx, unsigned long val)
 	}
 }
 
+static int any_pmc_overflown(struct cpu_hw_events *cpuhw)
+{
+	int i, idx;
+
+	for (i = 0; i < cpuhw->n_events; i++) {
+		idx = cpuhw->event[i]->hw.idx;
+		if ((idx) && ((int)read_pmc(idx) < 0))
+			return idx;
+	}
+
+	return 0;
+}
+
 /* Called from sysrq_handle_showregs() */
 void perf_event_print_debug(void)
 {
@@ -1272,11 +1285,13 @@ static void power_pmu_disable(struct pmu *pmu)
 
 		/*
 		 * Set the 'freeze counters' bit, clear EBE/BHRBA/PMCC/PMAO/FC56
+		 * Also clear PMXE to disable PMI's getting triggered in some
+		 * corner cases during PMU disable.
 		 */
 		val  = mmcr0 = mfspr(SPRN_MMCR0);
 		val |= MMCR0_FC;
 		val &= ~(MMCR0_EBE | MMCR0_BHRBA | MMCR0_PMCC | MMCR0_PMAO |
-			 MMCR0_FC56);
+			 MMCR0_PMXE | MMCR0_FC56);
 		/* Set mmcr0 PMCCEXT for p10 */
 		if (ppmu->flags & PPMU_ARCH_31)
 			val |= MMCR0_PMCCEXT;
@@ -1290,6 +1305,24 @@ static void power_pmu_disable(struct pmu *pmu)
 		mb();
 		isync();
 
+		/*
+		 * Some corner cases could clear the PMU counter overflow
+		 * while a masked PMI is pending. One of such case is
+		 * when a PMI happens during interrupt replay and perf
+		 * counter values gets cleared by PMU callbacks before
+		 * replay.
+		 *
+		 * If any of PMC corresponding to the active PMU events is
+		 * overflown, disable the interrupt by clearing the paca
+		 * bit for PMI since we are disabling the PMU now.
+		 * Otherwise provide a warning if there is PMI pending, but
+		 * no counter is found overflown.
+		 */
+		if (any_pmc_overflown(cpuhw))
+			clear_pmi_irq_pending();
+		else
+			WARN_ON(pmi_irq_pending());
+
 		val = mmcra = cpuhw->mmcr.mmcra;
 
 		/*
@@ -1381,6 +1414,15 @@ static void power_pmu_enable(struct pmu *pmu)
 	 * (possibly updated for removal of events).
 	 */
 	if (!cpuhw->n_added) {
+		/*
+		 * If there is any active event with an overflown PMC
+		 * value, Set back PACA_IRQ_PMI which would have got
+		 * cleared in power_pmu_disable.
+		 */
+		hard_irq_disable();
+		if (any_pmc_overflown(cpuhw))
+			set_pmi_irq_pending();
+
 		mtspr(SPRN_MMCRA, cpuhw->mmcr.mmcra & ~MMCRA_SAMPLE_ENABLE);
 		mtspr(SPRN_MMCR1, cpuhw->mmcr.mmcr1);
 		if (ppmu->flags & PPMU_ARCH_31)
@@ -2336,6 +2378,12 @@ static void __perf_event_interrupt(struct pt_regs *regs)
 				break;
 			}
 		}
+		/*
+		 * Clear PACA_IRQ_PMI in case it was set by
+		 * set_pmi_irq_pending() when PMU was enabled
+		 * after accounting for interrupts.
+		 */
+		clear_pmi_irq_pending();
 		if (!active)
 			/* reset non active counters that have overflowed */
 			write_pmc(i + 1, 0);
@@ -2355,6 +2403,15 @@ 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