Merge tag 'nmi.2023.02.14a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull x86 NMI diagnostics from Paul McKenney:
 "Add diagnostics to the x86 NMI handler to help detect NMI-handler bugs
  on the one hand and failing hardware on the other"

* tag 'nmi.2023.02.14a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  x86/nmi: Print reasons why backtrace NMIs are ignored
  x86/nmi: Accumulate NMI-progress evidence in exc_nmi()

arch/x86/kernel/nmi.c

@@ -69,6 +69,15 @@ struct nmi_stats {
 	unsigned int unknown;
 	unsigned int external;
 	unsigned int swallow;
+	unsigned long recv_jiffies;
+	unsigned long idt_seq;
+	unsigned long idt_nmi_seq;
+	unsigned long idt_ignored;
+	atomic_long_t idt_calls;
+	unsigned long idt_seq_snap;
+	unsigned long idt_nmi_seq_snap;
+	unsigned long idt_ignored_snap;
+	long idt_calls_snap;
 };
 
 static DEFINE_PER_CPU(struct nmi_stats, nmi_stats);
@@ -479,12 +488,15 @@ static DEFINE_PER_CPU(unsigned long, nmi_dr7);
 DEFINE_IDTENTRY_RAW(exc_nmi)
 {
 	irqentry_state_t irq_state;
+	struct nmi_stats *nsp = this_cpu_ptr(&nmi_stats);
 
 	/*
 	 * Re-enable NMIs right here when running as an SEV-ES guest. This might
 	 * cause nested NMIs, but those can be handled safely.
 	 */
 	sev_es_nmi_complete();
+	if (IS_ENABLED(CONFIG_NMI_CHECK_CPU))
+		arch_atomic_long_inc(&nsp->idt_calls);
 
 	if (IS_ENABLED(CONFIG_SMP) && arch_cpu_is_offline(smp_processor_id()))
 		return;
@@ -495,6 +507,11 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 	}
 	this_cpu_write(nmi_state, NMI_EXECUTING);
 	this_cpu_write(nmi_cr2, read_cr2());
+	if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) {
+		WRITE_ONCE(nsp->idt_seq, nsp->idt_seq + 1);
+		WARN_ON_ONCE(!(nsp->idt_seq & 0x1));
+		WRITE_ONCE(nsp->recv_jiffies, jiffies);
+	}
 nmi_restart:
 
 	/*
@@ -509,8 +526,19 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 
 	inc_irq_stat(__nmi_count);
 
-	if (!ignore_nmis)
+	if (IS_ENABLED(CONFIG_NMI_CHECK_CPU) && ignore_nmis) {
+		WRITE_ONCE(nsp->idt_ignored, nsp->idt_ignored + 1);
+	} else if (!ignore_nmis) {
+		if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) {
+			WRITE_ONCE(nsp->idt_nmi_seq, nsp->idt_nmi_seq + 1);
+			WARN_ON_ONCE(!(nsp->idt_nmi_seq & 0x1));
+		}
 		default_do_nmi(regs);
+		if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) {
+			WRITE_ONCE(nsp->idt_nmi_seq, nsp->idt_nmi_seq + 1);
+			WARN_ON_ONCE(nsp->idt_nmi_seq & 0x1);
+		}
+	}
 
 	irqentry_nmi_exit(regs, irq_state);
 
@@ -525,6 +553,11 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 
 	if (user_mode(regs))
 		mds_user_clear_cpu_buffers();
+	if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) {
+		WRITE_ONCE(nsp->idt_seq, nsp->idt_seq + 1);
+		WARN_ON_ONCE(nsp->idt_seq & 0x1);
+		WRITE_ONCE(nsp->recv_jiffies, jiffies);
+	}
 }
 
 #if defined(CONFIG_X86_64) && IS_ENABLED(CONFIG_KVM_INTEL)
@@ -537,6 +570,79 @@ DEFINE_IDTENTRY_RAW(exc_nmi_noist)
 EXPORT_SYMBOL_GPL(asm_exc_nmi_noist);
 #endif
 
+#ifdef CONFIG_NMI_CHECK_CPU
+
+static char *nmi_check_stall_msg[] = {
+/*									*/
+/* +--------- nsp->idt_seq_snap & 0x1: CPU is in NMI handler.		*/
+/* | +------ cpu_is_offline(cpu)					*/
+/* | | +--- nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls):	*/
+/* | | |	NMI handler has been invoked.				*/
+/* | | |								*/
+/* V V V								*/
+/* 0 0 0 */ "NMIs are not reaching exc_nmi() handler",
+/* 0 0 1 */ "exc_nmi() handler is ignoring NMIs",
+/* 0 1 0 */ "CPU is offline and NMIs are not reaching exc_nmi() handler",
+/* 0 1 1 */ "CPU is offline and exc_nmi() handler is legitimately ignoring NMIs",
+/* 1 0 0 */ "CPU is in exc_nmi() handler and no further NMIs are reaching handler",
+/* 1 0 1 */ "CPU is in exc_nmi() handler which is legitimately ignoring NMIs",
+/* 1 1 0 */ "CPU is offline in exc_nmi() handler and no more NMIs are reaching exc_nmi() handler",
+/* 1 1 1 */ "CPU is offline in exc_nmi() handler which is legitimately ignoring NMIs",
+};
+
+void nmi_backtrace_stall_snap(const struct cpumask *btp)
+{
+	int cpu;
+	struct nmi_stats *nsp;
+
+	for_each_cpu(cpu, btp) {
+		nsp = per_cpu_ptr(&nmi_stats, cpu);
+		nsp->idt_seq_snap = READ_ONCE(nsp->idt_seq);
+		nsp->idt_nmi_seq_snap = READ_ONCE(nsp->idt_nmi_seq);
+		nsp->idt_ignored_snap = READ_ONCE(nsp->idt_ignored);
+		nsp->idt_calls_snap = atomic_long_read(&nsp->idt_calls);
+	}
+}
+
+void nmi_backtrace_stall_check(const struct cpumask *btp)
+{
+	int cpu;
+	int idx;
+	unsigned long nmi_seq;
+	unsigned long j = jiffies;
+	char *modp;
+	char *msgp;
+	char *msghp;
+	struct nmi_stats *nsp;
+
+	for_each_cpu(cpu, btp) {
+		nsp = per_cpu_ptr(&nmi_stats, cpu);
+		modp = "";
+		msghp = "";
+		nmi_seq = READ_ONCE(nsp->idt_nmi_seq);
+		if (nsp->idt_nmi_seq_snap + 1 == nmi_seq && (nmi_seq & 0x1)) {
+			msgp = "CPU entered NMI handler function, but has not exited";
+		} else if ((nsp->idt_nmi_seq_snap & 0x1) != (nmi_seq & 0x1)) {
+			msgp = "CPU is handling NMIs";
+		} else {
+			idx = ((nsp->idt_seq_snap & 0x1) << 2) |
+			      (cpu_is_offline(cpu) << 1) |
+			      (nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls));
+			msgp = nmi_check_stall_msg[idx];
+			if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1))
+				modp = ", but OK because ignore_nmis was set";
+			if (nmi_seq & ~0x1)
+				msghp = " (CPU currently in NMI handler function)";
+			else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
+				msghp = " (CPU exited one NMI handler function)";
+		}
+		pr_alert("%s: CPU %d: %s%s%s, last activity: %lu jiffies ago.\n",
+			 __func__, cpu, msgp, modp, msghp, j - READ_ONCE(nsp->recv_jiffies));
+	}
+}
+
+#endif
+
 void stop_nmi(void)
 {
 	ignore_nmis++;

include/linux/nmi.h

@@ -214,4 +214,12 @@ int proc_watchdog_cpumask(struct ctl_table *, int, void *, size_t *, loff_t *);
 #include <asm/nmi.h>
 #endif
 
+#ifdef CONFIG_NMI_CHECK_CPU
+void nmi_backtrace_stall_snap(const struct cpumask *btp);
+void nmi_backtrace_stall_check(const struct cpumask *btp);
+#else
+static inline void nmi_backtrace_stall_snap(const struct cpumask *btp) {}
+static inline void nmi_backtrace_stall_check(const struct cpumask *btp) {}
+#endif
+
 #endif

lib/Kconfig.debug

@@ -1562,6 +1562,17 @@ config TRACE_IRQFLAGS_NMI
 	depends on TRACE_IRQFLAGS
 	depends on TRACE_IRQFLAGS_NMI_SUPPORT
 
+config NMI_CHECK_CPU
+	bool "Debugging for CPUs failing to respond to backtrace requests"
+	depends on DEBUG_KERNEL
+	depends on X86
+	default n
+	help
+	  Enables debug prints when a CPU fails to respond to a given
+	  backtrace NMI.  These prints provide some reasons why a CPU
+	  might legitimately be failing to respond, for example, if it
+	  is offline of if ignore_nmis is set.
+
 config DEBUG_IRQFLAGS
 	bool "Debug IRQ flag manipulation"
 	help

lib/nmi_backtrace.c

@@ -64,6 +64,7 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
 	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
 		pr_info("Sending NMI from CPU %d to CPUs %*pbl:\n",
 			this_cpu, nr_cpumask_bits, to_cpumask(backtrace_mask));
+		nmi_backtrace_stall_snap(to_cpumask(backtrace_mask));
 		raise(to_cpumask(backtrace_mask));
 	}
 
@@ -74,6 +75,7 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
 		mdelay(1);
 		touch_softlockup_watchdog();
 	}
+	nmi_backtrace_stall_check(to_cpumask(backtrace_mask));
 
 	/*
 	 * Force flush any remote buffers that might be stuck in IRQ context