powerpc/64s/interrupt: Perf NMI should not take normal exit path

NMI interrupts should exit with EXCEPTION_RESTORE_REGS not with
interrupt_return_srr, which is what the perf NMI handler currently does.
This breaks if a PMI hits after interrupt_exit_user_prepare_main() has
switched the context tracking to user mode, then the CT_WARN_ON() in
interrupt_exit_kernel_prepare() fires because it returns to kernel with
context set to user.

This could possibly be solved by soft-disabling PMIs in the exit path,
but that reduces our ability to profile that code. The warning could be
removed, but it's potentially useful.

All other NMIs and soft-NMIs return using EXCEPTION_RESTORE_REGS, so
this makes perf interrupts consistent with that and seems like the best
fix.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Squash in fixups from Nick]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221006140413.126443-3-npiggin@gmail.com

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

@@ -2357,9 +2357,21 @@ EXC_VIRT_END(performance_monitor, 0x4f00, 0x20)
 EXC_COMMON_BEGIN(performance_monitor_common)
 	GEN_COMMON performance_monitor
 	addi	r3,r1,STACK_FRAME_OVERHEAD
-	bl	performance_monitor_exception
+	lbz	r4,PACAIRQSOFTMASK(r13)
+	cmpdi	r4,IRQS_ENABLED
+	bne	1f
+	bl	performance_monitor_exception_async
 	b	interrupt_return_srr
+1:
+	bl	performance_monitor_exception_nmi
+	/* Clear MSR_RI before setting SRR0 and SRR1. */
+	li	r9,0
+	mtmsrd	r9,1
 
+	kuap_kernel_restore r9, r10
+
+	EXCEPTION_RESTORE_REGS hsrr=0
+	RFI_TO_KERNEL
 
 /**
  * Interrupt 0xf20 - Vector Unavailable Interrupt.

arch/powerpc/kernel/interrupt.c

@@ -377,13 +377,15 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
 	 * CT_WARN_ON comes here via program_check_exception, so avoid
 	 * recursion.
 	 *
-	 * Skip the assertion on PMIs to work around a problem caused by NMI
-	 * PMIs incorrectly taking this interrupt return path, it's possible
-	 * for this to hit after interrupt exit to user switches context to
-	 * user. See also the comment in the performance monitor handler in
-	 * exceptions-64e/s.S
+	 * Skip the assertion on PMIs on 64e to work around a problem caused
+	 * by NMI PMIs incorrectly taking this interrupt return path, it's
+	 * possible for this to hit after interrupt exit to user switches
+	 * context to user. See also the comment in the performance monitor
+	 * handler in exceptions-64e.S
 	 */
-	if (TRAP(regs) != INTERRUPT_PROGRAM && TRAP(regs) != INTERRUPT_PERFMON)
+	if (!IS_ENABLED(CONFIG_PPC_BOOK3E_64) &&
+	    TRAP(regs) != INTERRUPT_PROGRAM &&
+	    TRAP(regs) != INTERRUPT_PERFMON)
 		CT_WARN_ON(ct_state() == CONTEXT_USER);
 
 	kuap = kuap_get_and_assert_locked();