Commit 525445ef authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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()
parents 192a5e0a 344da544

arch/x86/kernel/nmi.c

+107 −1
Original line number Diff line number Diff line
@@ -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

+8 −0
Original line number Diff line number Diff line
@@ -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

+11 −0
Original line number Diff line number Diff line
@@ -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

+2 −0
Original line number Diff line number Diff line
@@ -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