powerpc/64s/exception: Soft NMI interrupt should not use ret_from_except

The soft NMI handler does not reconcile interrupt state, so it should
not return via the normal ret_from_except path. Return like other NMIs,
using the EXCEPTION_RESTORE_REGS macro.

This becomes important when the scv interrupt is implemented, which
must handle soft-masked interrupts that have r13 set to something
other than the PACA -- returning to kernel in this case must restore
r13.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-23-npiggin@gmail.com

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

@@ -2731,6 +2731,7 @@ EXC_VIRT_NONE(0x5800, 0x100)
 INT_DEFINE_BEGIN(soft_nmi)
 	IVEC=0x900
 	ISTACK=0
+	IRECONCILE=0	/* Soft-NMI may fire under local_irq_disable */
 INT_DEFINE_END(soft_nmi)
 
 /*
@@ -2749,9 +2750,36 @@ EXC_COMMON_BEGIN(soft_nmi_common)
 	subi	r1,r1,INT_FRAME_SIZE
 	__GEN_COMMON_BODY soft_nmi
 	bl	save_nvgprs
+
+	/*
+	 * Set IRQS_ALL_DISABLED and save PACAIRQHAPPENED (see
+	 * system_reset_common)
+	 */
+	li	r10,IRQS_ALL_DISABLED
+	stb	r10,PACAIRQSOFTMASK(r13)
+	lbz	r10,PACAIRQHAPPENED(r13)
+	std	r10,_DAR(r1)
+	ori	r10,r10,PACA_IRQ_HARD_DIS
+	stb	r10,PACAIRQHAPPENED(r13)
+
 	addi	r3,r1,STACK_FRAME_OVERHEAD
 	bl	soft_nmi_interrupt
-	b	ret_from_except
+
+	/* Clear MSR_RI before setting SRR0 and SRR1. */
+	li	r9,0
+	mtmsrd	r9,1
+
+	/*
+	 * Restore soft mask settings.
+	 */
+	ld	r10,_DAR(r1)
+	stb	r10,PACAIRQHAPPENED(r13)
+	ld	r10,SOFTE(r1)
+	stb	r10,PACAIRQSOFTMASK(r13)
+
+	kuap_restore_amr r10
+	EXCEPTION_RESTORE_REGS hsrr=0
+	RFI_TO_KERNEL
 
 #endif /* CONFIG_PPC_WATCHDOG */