diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index e16eb2a33173..6ff3cf506088 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -191,6 +191,37 @@ exception_marker:
 	ori	reg,reg,(label)@l;	/* virt addr of handler ... */
 #endif
 
+/*
+ * Equal to EXCEPTION_PROLOG_PSERIES, except that it forces 64bit mode.
+ * The firmware calls the registered system_reset_fwnmi and
+ * machine_check_fwnmi handlers in 32bit mode if the cpu happens to run
+ * a 32bit application at the time of the event.
+ * This firmware bug is present on POWER4 and JS20.
+ */
+#define EXCEPTION_PROLOG_PSERIES_FORCE_64BIT(area, label)		\
+	mfspr	r13,SPRN_SPRG3;		/* get paca address into r13 */	\
+	std	r9,area+EX_R9(r13);	/* save r9 - r12 */		\
+	std	r10,area+EX_R10(r13);					\
+	std	r11,area+EX_R11(r13);					\
+	std	r12,area+EX_R12(r13);					\
+	mfspr	r9,SPRN_SPRG1;						\
+	std	r9,area+EX_R13(r13);					\
+	mfcr	r9;							\
+	clrrdi	r12,r13,32;		/* get high part of &label */	\
+	mfmsr	r10;							\
+	/* force 64bit mode */						\
+	li	r11,5;			/* MSR_SF_LG|MSR_ISF_LG */	\
+	rldimi	r10,r11,61,0;		/* insert into top 3 bits */	\
+	/* done 64bit mode */						\
+	mfspr	r11,SPRN_SRR0;		/* save SRR0 */			\
+	LOAD_HANDLER(r12,label)						\
+	ori	r10,r10,MSR_IR|MSR_DR|MSR_RI;				\
+	mtspr	SPRN_SRR0,r12;						\
+	mfspr	r12,SPRN_SRR1;		/* and SRR1 */			\
+	mtspr	SPRN_SRR1,r10;						\
+	rfid;								\
+	b	.	/* prevent speculative execution */
+
 #define EXCEPTION_PROLOG_PSERIES(area, label)				\
 	mfspr	r13,SPRN_SPRG3;		/* get paca address into r13 */	\
 	std	r9,area+EX_R9(r13);	/* save r9 - r12 */		\
@@ -604,14 +635,14 @@ slb_miss_user_pseries:
 system_reset_fwnmi:
 	HMT_MEDIUM
 	mtspr	SPRN_SPRG1,r13		/* save r13 */
-	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, system_reset_common)
+	EXCEPTION_PROLOG_PSERIES_FORCE_64BIT(PACA_EXGEN, system_reset_common)
 
 	.globl machine_check_fwnmi
       .align 7
 machine_check_fwnmi:
 	HMT_MEDIUM
 	mtspr	SPRN_SPRG1,r13		/* save r13 */
-	EXCEPTION_PROLOG_PSERIES(PACA_EXMC, machine_check_common)
+	EXCEPTION_PROLOG_PSERIES_FORCE_64BIT(PACA_EXMC, machine_check_common)
 
 #ifdef CONFIG_PPC_ISERIES
 /***  ISeries-LPAR interrupt handlers ***/