powerpc/64s: Disallow system reset vs system reset reentrancy

In preparation for using a dedicated stack for system reset interrupts,
prevent a nested system reset from recovering, in order to simplify
code that is called in crash/debug path. This allows a system reset
interrupt to just use the base stack pointer.

Keep an in_nmi nesting counter similarly to the in_mce counter. Consider
the interrrupt non-recoverable if it is taken inside another system
reset.

Interrupt nesting could be allowed similarly to MCE, but system reset
is a special case that's not for normal operation, so simplicity wins
until there is requirement for nested system reset interrupts.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/exception-64s.h

@@ -275,6 +275,12 @@ END_FTR_SECTION_NESTED(ftr,ftr,943)
 
 #endif
 
+/* Do not enable RI */
+#define EXCEPTION_PROLOG_PSERIES_NORI(area, label, h, extra, vec)	\
+	EXCEPTION_PROLOG_0(area);					\
+	EXCEPTION_PROLOG_1(area, extra, vec);				\
+	EXCEPTION_PROLOG_PSERIES_1_NORI(label, h);
+
 
 #define __KVM_HANDLER(area, h, n)					\
 	BEGIN_FTR_SECTION_NESTED(947)					\

arch/powerpc/include/asm/paca.h

@@ -187,12 +187,15 @@ struct paca_struct {
 #ifdef CONFIG_PPC_BOOK3S_64
 	/* Exclusive emergency stack pointer for machine check exception. */
 	void *mc_emergency_sp;
+
+	u16 in_nmi;			/* In nmi handler */
+
 	/*
 	 * Flag to check whether we are in machine check early handler
 	 * and already using emergency stack.
 	 */
 	u16 in_mce;
-	u8 hmi_event_available;		 /* HMI event is available */
+	u8 hmi_event_available;		/* HMI event is available */
 #endif
 
 	/* Stuff for accurate time accounting */

arch/powerpc/kernel/asm-offsets.c

@@ -235,6 +235,7 @@ int main(void)
 #ifdef CONFIG_PPC_BOOK3S_64
 	OFFSET(PACAMCEMERGSP, paca_struct, mc_emergency_sp);
 	OFFSET(PACA_IN_MCE, paca_struct, in_mce);
+	OFFSET(PACA_IN_NMI, paca_struct, in_nmi);
 #endif
 	OFFSET(PACAHWCPUID, paca_struct, hw_cpu_id);
 	OFFSET(PACAKEXECSTATE, paca_struct, kexec_state);

arch/powerpc/kernel/exceptions-64s.S

@@ -116,7 +116,11 @@ EXC_VIRT_NONE(0x4000, 0x100)
 
 EXC_REAL_BEGIN(system_reset, 0x100, 0x100)
 	SET_SCRATCH0(r13)
-	EXCEPTION_PROLOG_PSERIES(PACA_EXNMI, system_reset_common, EXC_STD,
+	/*
+	 * MSR_RI is not enabled, because PACA_EXNMI and nmi stack is
+	 * being used, so a nested NMI exception would corrupt it.
+	 */
+	EXCEPTION_PROLOG_PSERIES_NORI(PACA_EXNMI, system_reset_common, EXC_STD,
 				 IDLETEST, 0x100)
 
 EXC_REAL_END(system_reset, 0x100, 0x100)
@@ -128,9 +132,31 @@ EXC_COMMON_BEGIN(system_reset_idle_common)
 #endif
 
 EXC_COMMON_BEGIN(system_reset_common)
+	/*
+	 * Increment paca->in_nmi then enable MSR_RI. SLB or MCE will be able
+	 * to recover, but nested NMI will notice in_nmi and not recover
+	 * because of the use of the NMI stack. in_nmi reentrancy is tested in
+	 * system_reset_exception.
+	 */
+	lhz	r10,PACA_IN_NMI(r13)
+	addi	r10,r10,1
+	sth	r10,PACA_IN_NMI(r13)
+	li	r10,MSR_RI
+	mtmsrd 	r10,1
+
 	EXCEPTION_COMMON(PACA_EXNMI, 0x100,
-			system_reset, system_reset_exception,
-			ret_from_except, ADD_NVGPRS;ADD_RECONCILE)
+			system_reset, system_reset_exception, 1f,
+			ADD_NVGPRS;ADD_RECONCILE)
+1: /* EXCEPTION_COMMON continues here */
+
+	/*
+	 * The stack is no longer in use, decrement in_nmi.
+	 */
+	lhz	r10,PACA_IN_NMI(r13)
+	subi	r10,r10,1
+	sth	r10,PACA_IN_NMI(r13)
+
+	b	ret_from_except
 
 #ifdef CONFIG_PPC_PSERIES
 /*
@@ -138,8 +164,9 @@ EXC_COMMON_BEGIN(system_reset_common)
  */
 TRAMP_REAL_BEGIN(system_reset_fwnmi)
 	SET_SCRATCH0(r13)		/* save r13 */
-	EXCEPTION_PROLOG_PSERIES(PACA_EXNMI, system_reset_common, EXC_STD,
-				 NOTEST, 0x100)
+	/* See comment at system_reset exception */
+	EXCEPTION_PROLOG_PSERIES_NORI(PACA_EXNMI, system_reset_common,
+						EXC_STD, NOTEST, 0x100)
 #endif /* CONFIG_PPC_PSERIES */
 
 

arch/powerpc/kernel/traps.c

@@ -282,11 +282,17 @@ void system_reset_exception(struct pt_regs *regs)
 	/* See if any machine dependent calls */
 	if (ppc_md.system_reset_exception) {
 		if (ppc_md.system_reset_exception(regs))
-			return;
+			goto out;
 	}
 
 	die("System Reset", regs, SIGABRT);
 
+out:
+#ifdef CONFIG_PPC_BOOK3S_64
+	BUG_ON(get_paca()->in_nmi == 0);
+	if (get_paca()->in_nmi > 1)
+		panic("Unrecoverable nested System Reset");
+#endif
 	/* Must die if the interrupt is not recoverable */
 	if (!(regs->msr & MSR_RI))
 		panic("Unrecoverable System Reset");

arch/powerpc/xmon/xmon.c

@@ -2236,6 +2236,7 @@ static void dump_one_paca(int cpu)
 	DUMP(p, emergency_sp, "p");
 #ifdef CONFIG_PPC_BOOK3S_64
 	DUMP(p, mc_emergency_sp, "p");
+	DUMP(p, in_nmi, "x");
 	DUMP(p, in_mce, "x");
 	DUMP(p, hmi_event_available, "x");
 #endif