Excerpts from Christophe Leroy's message of January 14, 2021 1:13 am:

Le 13/01/2021 à 08:32, Nicholas Piggin a écrit :

quoted

This moves the common NMI entry and exit code into the interrupt handler
wrappers.

This changes the behaviour of soft-NMI (watchdog) and HMI interrupts, and
also MCE interrupts on 64e, by adding missing parts of the NMI entry to
them.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
  arch/powerpc/include/asm/interrupt.h | 24 ++++++++++++++++
  arch/powerpc/kernel/mce.c            | 11 --------
  arch/powerpc/kernel/traps.c          | 42 +++++-----------------------
  arch/powerpc/kernel/watchdog.c       | 10 +++----
  4 files changed, 35 insertions(+), 52 deletions(-)

diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
index e278dffe7657..01192e213f9a 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h

@@ -95,14 +95,38 @@ static inline void interrupt_async_exit_prepare(struct pt_regs *regs, struct int
  }
  
  struct interrupt_nmi_state {
+#ifdef CONFIG_PPC64
+	u8 ftrace_enabled;
+#endif
  };
  
  static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct interrupt_nmi_state *state)
  {
+#ifdef CONFIG_PPC64
+	state->ftrace_enabled = this_cpu_get_ftrace_enabled();
+	this_cpu_set_ftrace_enabled(0);
+#endif
+
+	/*
+	 * Do not use nmi_enter() for pseries hash guest taking a real-mode
+	 * NMI because not everything it touches is within the RMA limit.
+	 */
+	if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64) ||
+			!firmware_has_feature(FW_FEATURE_LPAR) ||
+			radix_enabled() || (mfmsr() & MSR_DR))
+		nmi_enter();
  }
  
  static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct interrupt_nmi_state *state)
  {
+	if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64) ||
+			!firmware_has_feature(FW_FEATURE_LPAR) ||
+			radix_enabled() || (mfmsr() & MSR_DR))
+		nmi_exit();
+
+#ifdef CONFIG_PPC64
+	this_cpu_set_ftrace_enabled(state->ftrace_enabled);
+#endif
  }
  
  /**

diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index 54269947113d..51456217ec40 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c

@@ -592,12 +592,6 @@ EXPORT_SYMBOL_GPL(machine_check_print_event_info);
  DEFINE_INTERRUPT_HANDLER_NMI(machine_check_early)
  {
  	long handled = 0;
-	u8 ftrace_enabled = this_cpu_get_ftrace_enabled();
-
-	this_cpu_set_ftrace_enabled(0);
-	/* Do not use nmi_enter/exit for pseries hpte guest */
-	if (radix_enabled() || !firmware_has_feature(FW_FEATURE_LPAR))
-		nmi_enter();
  
  	hv_nmi_check_nonrecoverable(regs);
  

@@ -607,11 +601,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(machine_check_early)
  	if (ppc_md.machine_check_early)
  		handled = ppc_md.machine_check_early(regs);
  
-	if (radix_enabled() || !firmware_has_feature(FW_FEATURE_LPAR))
-		nmi_exit();
-
-	this_cpu_set_ftrace_enabled(ftrace_enabled);
-
  	return handled;
  }
  

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index b4f23e871a68..43d23232ef5c 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c

@@ -435,11 +435,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(system_reset_exception)
  {
  	unsigned long hsrr0, hsrr1;
  	bool saved_hsrrs = false;
-	u8 ftrace_enabled = this_cpu_get_ftrace_enabled();
-
-	this_cpu_set_ftrace_enabled(0);
-
-	nmi_enter();
  
  	/*
  	 * System reset can interrupt code where HSRRs are live and MSR[RI]=1.

@@ -511,10 +506,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(system_reset_exception)
  		mtspr(SPRN_HSRR1, hsrr1);
  	}
  
-	nmi_exit();
-
-	this_cpu_set_ftrace_enabled(ftrace_enabled);
-
  	/* What should we do here? We could issue a shutdown or hard reset. */
  
  	return 0;

@@ -792,6 +783,12 @@ int machine_check_generic(struct pt_regs *regs)
  #endif /* everything else */
  
  
+/*
+ * BOOK3S_64 does not call this handler as a non-maskable interrupt
+ * (it uses its own early real-mode handler to handle the MCE proper
+ * and then raises irq_work to call this handler when interrupts are
+ * enabled).
+ */
  #ifdef CONFIG_PPC_BOOK3S_64
  DEFINE_INTERRUPT_HANDLER_ASYNC(machine_check_exception)
  #else

@@ -800,20 +797,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(machine_check_exception)
  {
  	int recover = 0;
  
-	/*
-	 * BOOK3S_64 does not call this handler as a non-maskable interrupt
-	 * (it uses its own early real-mode handler to handle the MCE proper
-	 * and then raises irq_work to call this handler when interrupts are
-	 * enabled).
-	 *
-	 * This is silly. The BOOK3S_64 should just call a different function
-	 * rather than expecting semantics to magically change. Something
-	 * like 'non_nmi_machine_check_exception()', perhaps?
-	 */
-	const bool nmi = !IS_ENABLED(CONFIG_PPC_BOOK3S_64);
-
-	if (nmi) nmi_enter();
-
  	__this_cpu_inc(irq_stat.mce_exceptions);
  
  	add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);

@@ -838,24 +821,17 @@ DEFINE_INTERRUPT_HANDLER_NMI(machine_check_exception)
  	if (check_io_access(regs))
  		goto bail;
  
-	if (nmi) nmi_exit();
-

IIRC, not doing the nmi_exit() before the die() is problematic.

See 
https://github.com/linuxppc/linux/commit/daf00ae71dad8aa05965713c62558aeebf2df48e#diff-70077148c383252ca949063eaf1b0250620e4607b43f4ef3fd2d8f448a83ab0a

Yes good catch. Maybe putting it into a nmi_die() or having die 
explicitly check for the NMI case might be the go.

Thanks,
Nick