powerpc/64s/exception: treat NIA below __end_interrupts as soft-masked

The scv instruction causes an interrupt which can enter the kernel with
MSR[EE]=1, thus allowing interrupts to hit at any time. These must not
be taken as normal interrupts, because they come from MSR[PR]=0 context,
and yet the kernel stack is not yet set up and r13 is not set to the
PACA).

Treat this as a soft-masked interrupt regardless of the soft masked
state. This does not affect behaviour yet, because currently all
interrupts are taken with MSR[EE]=0.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200611081203.995112-2-npiggin@gmail.com

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

@@ -508,8 +508,24 @@ DEFINE_FIXED_SYMBOL(\name\()_common_real)
 
 .macro __GEN_COMMON_BODY name
 	.if IMASK
+		.if ! ISTACK
+		.error "No support for masked interrupt to use custom stack"
+		.endif
+
+		/* If coming from user, skip soft-mask tests. */
+		andi.	r10,r12,MSR_PR
+		bne	2f
+
+		/* Kernel code running below __end_interrupts is implicitly
+		 * soft-masked */
+		LOAD_HANDLER(r10, __end_interrupts)
+		cmpld	r11,r10
+		li	r10,IMASK
+		blt-	1f
+
+		/* Test the soft mask state against our interrupt's bit */
 		lbz	r10,PACAIRQSOFTMASK(r13)
-		andi.	r10,r10,IMASK
+1:		andi.	r10,r10,IMASK
 		/* Associate vector numbers with bits in paca->irq_happened */
 		.if IVEC == 0x500 || IVEC == 0xea0
 		li	r10,PACA_IRQ_EE
@@ -540,7 +556,7 @@ DEFINE_FIXED_SYMBOL(\name\()_common_real)
 
 	.if ISTACK
 	andi.	r10,r12,MSR_PR		/* See if coming from user	*/
-	mr	r10,r1			/* Save r1			*/
+2:	mr	r10,r1			/* Save r1			*/
 	subi	r1,r1,INT_FRAME_SIZE	/* alloc frame on kernel stack	*/
 	beq-	100f
 	ld	r1,PACAKSAVE(r13)	/* kernel stack to use		*/
@@ -2838,7 +2854,8 @@ masked_interrupt:
 	ld	r10,PACA_EXGEN+EX_R10(r13)
 	ld	r11,PACA_EXGEN+EX_R11(r13)
 	ld	r12,PACA_EXGEN+EX_R12(r13)
-	/* returns to kernel where r13 must be set up, so don't restore it */
+	ld	r13,PACA_EXGEN+EX_R13(r13)
+	/* May return to masked low address where r13 is not set up */
 	.if \hsrr
 	HRFI_TO_KERNEL
 	.else
@@ -2997,6 +3014,10 @@ EXC_COMMON_BEGIN(ppc64_runlatch_on_trampoline)
 
 USE_FIXED_SECTION(virt_trampolines)
 	/*
+	 * All code below __end_interrupts is treated as soft-masked. If
+	 * any code runs here with MSR[EE]=1, it must then cope with pending
+	 * soft interrupt being raised (i.e., by ensuring it is replayed).
+	 *
 	 * The __end_interrupts marker must be past the out-of-line (OOL)
 	 * handlers, so that they are copied to real address 0x100 when running
 	 * a relocatable kernel. This ensures they can be reached from the short