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Subject[PATCH x86/nmi 1/2] x86/nmi: Accumulate NMI-progress evidence in exc_nmi()
Date
CPUs ignoring NMIs is often a sign of those CPUs going bad, but there
are quite a few other reasons why a CPU might ignore NMIs. Therefore,
accumulate evidence within exc_nmi() as to what might be preventing a
given CPU from responding to an NMI.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <x86@kernel.org>
---
arch/x86/kernel/nmi.c | 31 ++++++++++++++++++++++++++++++-
lib/Kconfig.debug | 11 +++++++++++
2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index cec0bfa3bc04f..4f1651dc65b3a 100644
--- a/arch/x86/kernel/nmi.c
+++ b/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 +526,15 @@ 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) {
+ WRITE_ONCE(nsp->idt_nmi_seq, nsp->idt_nmi_seq + 1);
+ WARN_ON_ONCE(!(nsp->idt_nmi_seq & 0x1));
default_do_nmi(regs);
+ 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 +549,11 @@ 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)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 881c3f84e88a3..dad99197d2695 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1552,6 +1552,17 @@ 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
--
2.31.1.189.g2e36527f23
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