powerpc/booke64: Use SPRG0/3 scratch for bolted TLB miss & crit int

Embedded.Hypervisor category defines GSPRG0..3 physical registers for guests.
Avoid SPRG4-7 usage as scratch in host exception handlers, otherwise guest
SPRG4-7 registers will be clobbered.
For bolted TLB miss exception handlers, which is the version currently
supported by KVM, use SPRN_SPRG_GEN_SCRATCH aka SPRG0 instead of
SPRN_SPRG_TLB_SCRATCH aka SPRG6. Keep using TLB PACA slots to fit in one
64-byte cache line.
For critical exception handlers use SPRG3 instead of SPRG7. Provide a routine
to store and restore user-visible SPRGs. This will be subsequently used
to restore VDSO information in SPRG3. Add EX_R13 to paca slots to free up
SPRG3 and change the critical exception epilog to use it.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/tlb_low_64e.S

@@ -40,7 +40,7 @@
  **********************************************************************/
 
 .macro tlb_prolog_bolted intnum addr
-	mtspr	SPRN_SPRG_TLB_SCRATCH,r13
+	mtspr	SPRN_SPRG_GEN_SCRATCH,r13
 	mfspr	r13,SPRN_SPRG_PACA
 	std	r10,PACA_EXTLB+EX_TLB_R10(r13)
 	mfcr	r10
@@ -69,7 +69,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_EMB_HV)
 	ld	r15,PACA_EXTLB+EX_TLB_R15(r13)
 	TLB_MISS_RESTORE_STATS_BOLTED
 	ld	r16,PACA_EXTLB+EX_TLB_R16(r13)
-	mfspr	r13,SPRN_SPRG_TLB_SCRATCH
+	mfspr	r13,SPRN_SPRG_GEN_SCRATCH
 .endm
 
 /* Data TLB miss */