powerpc/book3s: Fix CFAR clobbering issue in machine check handler.

While checking powersaving mode in machine check handler at 0x200, we
clobber CFAR register. Fix it by saving and restoring it during beq/bgt.

Signed-off-by: Mahesh Salgaonkar <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/include/asm/exception-64s.h

···
 END_FTR_SECTION_NESTED(ftr,ftr,943)
 
 /*
+ * Set an SPR from a register if the CPU has the given feature
+ */
+#define OPT_SET_SPR(ra, spr, ftr)					\
+BEGIN_FTR_SECTION_NESTED(943)						\
+	mtspr	spr,ra;							\
+END_FTR_SECTION_NESTED(ftr,ftr,943)
+
+/*
  * Save a register to the PACA if the CPU has the given feature
  */
 #define OPT_SAVE_REG_TO_PACA(offset, ra, ftr)				\

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

···
 	 */
 	mfspr	r13,SPRN_SRR1
 	rlwinm.	r13,r13,47-31,30,31
+	OPT_GET_SPR(r13, SPRN_CFAR, CPU_FTR_CFAR)
 	beq	9f
 
+	mfspr	r13,SPRN_SRR1
+	rlwinm.	r13,r13,47-31,30,31
 	/* waking up from powersave (nap) state */
 	cmpwi	cr1,r13,2
 	/* Total loss of HV state is fatal. let's just stay stuck here */
+	OPT_GET_SPR(r13, SPRN_CFAR, CPU_FTR_CFAR)
 	bgt	cr1,.
 9:
+	OPT_SET_SPR(r13, SPRN_CFAR, CPU_FTR_CFAR)
 END_FTR_SECTION_IFSET(CPU_FTR_HVMODE | CPU_FTR_ARCH_206)
 #endif /* CONFIG_PPC_P7_NAP */
 	EXCEPTION_PROLOG_0(PACA_EXMC)