powerpc: Fix denormalized exception handler

The denormalized exception handler (denorm_exception_hv) has a couple
of bugs.  If the CONFIG_PPC_DENORMALISATION option is not selected,
or the HSRR1_DENORM bit is not set in HSRR1, we don't test whether the
interrupt occurred within a KVM guest.  On the other hand, if the
HSRR1_DENORM bit is set and CONFIG_PPC_DENORMALISATION is enabled,
we corrupt the CFAR and PPR.

To correct these problems, this replaces the open-coded version of
EXCEPTION_PROLOG_1 that is there currently, and that is missing the
saving of PPR and CFAR values to the PACA, with an instance of
EXCEPTION_PROLOG_1.  This adds an explicit KVMTEST after testing
whether the exception is one we can handle, and adds code to restore
the CFAR on exit.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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

@@ -367,11 +367,7 @@ denorm_exception_hv:
 	HMT_MEDIUM_PPR_DISCARD
 	mtspr	SPRN_SPRG_HSCRATCH0,r13
 	EXCEPTION_PROLOG_0(PACA_EXGEN)
-	std	r11,PACA_EXGEN+EX_R11(r13)
-	std	r12,PACA_EXGEN+EX_R12(r13)
-	mfspr	r9,SPRN_SPRG_HSCRATCH0
-	std	r9,PACA_EXGEN+EX_R13(r13)
-	mfcr	r9
+	EXCEPTION_PROLOG_1(PACA_EXGEN, NOTEST, 0x1500)
 
 #ifdef CONFIG_PPC_DENORMALISATION
 	mfspr	r10,SPRN_HSRR1
@@ -381,6 +377,7 @@ denorm_exception_hv:
 	bne+	denorm_assist
 #endif
 
+	KVMTEST(0x1500)
 	EXCEPTION_PROLOG_PSERIES_1(denorm_common, EXC_HV)
 	KVM_HANDLER_SKIP(PACA_EXGEN, EXC_STD, 0x1500)
 
@@ -501,6 +498,10 @@ denorm_done:
 	mtcrf	0x80,r9
 	ld	r9,PACA_EXGEN+EX_R9(r13)
 	RESTORE_PPR_PACA(PACA_EXGEN, r10)
+BEGIN_FTR_SECTION
+	ld	r10,PACA_EXGEN+EX_CFAR(r13)
+	mtspr	SPRN_CFAR,r10
+END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
 	ld	r10,PACA_EXGEN+EX_R10(r13)
 	ld	r11,PACA_EXGEN+EX_R11(r13)
 	ld	r12,PACA_EXGEN+EX_R12(r13)