[PATCH 2/2] powerpc: Fix crash at boot with CONFIG_PPC_RADIX_MMU=n

[Michael Ellerman <mpe@ellerman.id.au>]

Currently a kernel that is built with CONFIG_PPC_RADIX_MMU=n, and then
booted on a 64-bit Hash MMU system will crash on the first SLB miss,
typically with an oops something like:

  Unrecoverable exception 4100 at c000000000969504
  cpu 0x0: Vector: 4100  at [c000000000de78e0]
      pc: c000000000969504: memmap_init_zone+0x160/0x2dc
      lr: c0000000009694b0: memmap_init_zone+0x10c/0x2dc
  ...
  [c000000000de7b60] c000000000968ec8 init_currently_empty_zone+0x3c/0x11c (unreliable)
  [c000000000de7bf0] c000000000969bc0 free_area_init_node+0x540/0x688
  [c000000000de7cf0] c000000000c4b3b4 free_area_init_nodes+0x7b4/0x864
  [c000000000de7df0] c000000000c2fce0 paging_init+0x88/0xa4
  [c000000000de7e60] c000000000c2b49c setup_arch+0x29c/0x2ec
  [c000000000de7f00] c000000000c23b7c start_kernel+0x88/0x524
  [c000000000de7f90] c000000000008c60 start_here_common+0x20/0xa0

This is caused by the branch in slb_miss_realmode() that jumps directly
to the unrecoverable case when MMU_FTR_RADIX is set:

  BEGIN_MMU_FTR_SECTION
  	b	2f
  END_MMU_FTR_SECTION_IFSET(MMU_FTR_RADIX)

When CONFIG_PPC_RADIX_MMU=n, MMU_FTR_RADIX == 0, and so the test
becomes:

	(cur_cpu_spec->mmu_features & 0) == 0

Which is always true. This causes the branch to *not* be patched with a
nop, which is incorrect.

The root cause is my change to Aneesh's patch to make MMU_FTR_RADIX == 0
when CONFIG_PPC_RADIX_MMU=n, which was designed to allow the
radix_enabled() checks to compile out.

We can achieve the same result (in fact identical code generation) by
instead using MMU_FTRS_POSSIBLE and only adding MMU_FTR_RADIX to it when
CONFIG_PPC_RADIX_MMU=y.

Fixes: 418d145591b6 ("powerpc/mm/radix: Add MMU_FTR_RADIX")
Reported-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/include/asm/mmu.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
index a5e37c93700b..ad68da9344c8 100644
--- a/arch/powerpc/include/asm/mmu.h
+++ b/arch/powerpc/include/asm/mmu.h
@@ -91,11 +91,7 @@
 /*
  * Radix page table available
  */
-#ifdef CONFIG_PPC_RADIX_MMU
 #define MMU_FTR_RADIX                  ASM_CONST(0x80000000)
-#else
-#define MMU_FTR_RADIX                  ASM_CONST(0)
-#endif
 
 /* MMU feature bit sets for various CPUs */
 #define MMU_FTRS_DEFAULT_HPTE_ARCH_V2	\
@@ -128,7 +124,11 @@ enum {
 		MMU_FTR_USE_TLBRSRV | MMU_FTR_USE_PAIRED_MAS |
 		MMU_FTR_NO_SLBIE_B | MMU_FTR_16M_PAGE | MMU_FTR_TLBIEL |
 		MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_CI_LARGE_PAGE |
-		MMU_FTR_1T_SEGMENT | MMU_FTR_RADIX,
+		MMU_FTR_1T_SEGMENT |
+#ifdef CONFIG_PPC_RADIX_MMU
+		MMU_FTR_RADIX |
+#endif
+		0
 };
 
 static inline int mmu_has_feature(unsigned long feature)
-- 
2.5.0