powerpc/64s: machine check interrupt update NMI accounting

machine_check_early() is taken as an NMI, so nmi_enter() is used
there. machine_check_exception() is no longer taken as an NMI (it's
invoked via irq_work in the case a machine check hits in kernel mode),
so remove the nmi_enter() from that case.

In NMI context, hash faults don't try to refill the hash table, which
can lead to crashes accessing non-pinned kernel pages. System reset
still has this potential problem.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Drop change in show_regs() which breaks Book3E]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200508043408.886394-12-npiggin@gmail.com

arch/powerpc/kernel/mce.c

@@ -574,6 +574,9 @@ EXPORT_SYMBOL_GPL(machine_check_print_event_info);
 long machine_check_early(struct pt_regs *regs)
 {
 	long handled = 0;
+	bool nested = in_nmi();
+	if (!nested)
+		nmi_enter();
 
 	hv_nmi_check_nonrecoverable(regs);
 
@@ -582,6 +585,10 @@ long machine_check_early(struct pt_regs *regs)
 	 */
 	if (ppc_md.machine_check_early)
 		handled = ppc_md.machine_check_early(regs);
+
+	if (!nested)
+		nmi_exit();
+
 	return handled;
 }
 

arch/powerpc/kernel/traps.c

@@ -823,7 +823,19 @@ int machine_check_generic(struct pt_regs *regs)
 void machine_check_exception(struct pt_regs *regs)
 {
 	int recover = 0;
-	bool nested = in_nmi();
+	bool nested;
+
+	/*
+	 * 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). Set nested = true for this case, which just makes it avoid
+	 * the nmi_enter/exit.
+	 */
+	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) || in_nmi())
+		nested = true;
+	else
+		nested = false;
 	if (!nested)
 		nmi_enter();