powerpc/64s: system reset interrupt preserve HSRRs

Code that uses HSRR registers is not required to clear MSR[RI] by
convention, however the system reset NMI itself may use HSRR
registers (e.g., to call OPAL) and clobber them.

Rather than introduce the requirement to clear RI in order to use
HSRRs, have system reset interrupt save and restore HSRRs.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -435,14 +435,32 @@ void hv_nmi_check_nonrecoverable(struct pt_regs *regs)
 
 void system_reset_exception(struct pt_regs *regs)
 {
+	unsigned long hsrr0, hsrr1;
+	bool nested = in_nmi();
+	bool saved_hsrrs = false;
+
 	/*
 	 * Avoid crashes in case of nested NMI exceptions. Recoverability
 	 * is determined by RI and in_nmi
 	 */
-	bool nested = in_nmi();
 	if (!nested)
 		nmi_enter();
 
+	/*
+	 * System reset can interrupt code where HSRRs are live and MSR[RI]=1.
+	 * The system reset interrupt itself may clobber HSRRs (e.g., to call
+	 * OPAL), so save them here and restore them before returning.
+	 *
+	 * Machine checks don't need to save HSRRs, as the real mode handler
+	 * is careful to avoid them, and the regular handler is not delivered
+	 * as an NMI.
+	 */
+	if (cpu_has_feature(CPU_FTR_HVMODE)) {
+		hsrr0 = mfspr(SPRN_HSRR0);
+		hsrr1 = mfspr(SPRN_HSRR1);
+		saved_hsrrs = true;
+	}
+
 	hv_nmi_check_nonrecoverable(regs);
 
 	__this_cpu_inc(irq_stat.sreset_irqs);
@@ -492,6 +510,11 @@ void system_reset_exception(struct pt_regs *regs)
 	if (!(regs->msr & MSR_RI))
 		nmi_panic(regs, "Unrecoverable System Reset");
 
+	if (saved_hsrrs) {
+		mtspr(SPRN_HSRR0, hsrr0);
+		mtspr(SPRN_HSRR1, hsrr1);
+	}
+
 	if (!nested)
 		nmi_exit();