powerpc/64s: Remove EXSLB interrupt save area

SLB faults should not be taken while the PACA save areas are live, all
memory accesses should be fetches from the kernel text, and access to
PACA and the current stack, before C code is called or any other
accesses are made.

All of these have pinned SLBs so will not take a SLB fault. Therefore
EXSLB is not be required.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210208063406.331655-1-npiggin@gmail.com

arch/powerpc/include/asm/paca.h

@@ -109,8 +109,7 @@ struct paca_struct {
 	 */
 	/* used for most interrupts/exceptions */
 	u64 exgen[EX_SIZE] __attribute__((aligned(0x80)));
-	u64 exslb[EX_SIZE];	/* used for SLB/segment table misses
- 				 * on the linear mapping */
+
 	/* SLB related definitions */
 	u16 vmalloc_sllp;
 	u8 slb_cache_ptr;

arch/powerpc/kernel/asm-offsets.c

@@ -255,7 +255,6 @@ int main(void)
 #endif /* CONFIG_PPC_MM_SLICES */
 	OFFSET(PACA_EXGEN, paca_struct, exgen);
 	OFFSET(PACA_EXMC, paca_struct, exmc);
-	OFFSET(PACA_EXSLB, paca_struct, exslb);
 	OFFSET(PACA_EXNMI, paca_struct, exnmi);
 #ifdef CONFIG_PPC_PSERIES
 	OFFSET(PACALPPACAPTR, paca_struct, lppaca_ptr);

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

@@ -1412,13 +1412,9 @@ ALT_MMU_FTR_SECTION_END_IFCLR(MMU_FTR_TYPE_RADIX)
  *   on user-handler data structures.
  *
  *   KVM: Same as 0x300, DSLB must test for KVM guest.
- *
- * A dedicated save area EXSLB is used (XXX: but it actually need not be
- * these days, we could use EXGEN).
  */
 INT_DEFINE_BEGIN(data_access_slb)
 	IVEC=0x380
-	IAREA=PACA_EXSLB
 	IDAR=1
 	IKVM_SKIP=1
 	IKVM_REAL=1
@@ -1501,7 +1497,6 @@ ALT_MMU_FTR_SECTION_END_IFCLR(MMU_FTR_TYPE_RADIX)
  */
 INT_DEFINE_BEGIN(instruction_access_slb)
 	IVEC=0x480
-	IAREA=PACA_EXSLB
 	IISIDE=1
 	IDAR=1
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE