powerpc/mm: Fixup kernel read only mapping

With commit e58e87adc8bf9 ("powerpc/mm: Update _PAGE_KERNEL_RO") we
started using the ppp value 0b110 to map kernel readonly. But that
facility was only added as part of ISA 2.04. For earlier ISA version
only supported ppp bit value for readonly mapping is 0b011. (This
implies both user and kernel get mapped using the same ppp bit value for
readonly mapping.).
Update the code such that for earlier architecture version we use ppp
value 0b011 for readonly mapping. We don't differentiate between power5+
and power5 here and apply the new ppp bits only from power6 (ISA 2.05).
This keep the changes minimal.

This fixes issue with PS3 spu usage reported at
https://lkml.kernel.org/r/[e-mail]

Fixes: e58e87adc8bf9 ("powerpc/mm: Update _PAGE_KERNEL_RO")
Cc: [e-mail] # v4.7+
Tested-by: Geoff Levand <[e-mail]>
Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/include/asm/mmu.h

···
  */
 
 /*
+ * Kernel read only support.
+ * We added the ppp value 0b110 in ISA 2.04.
+ */
+#define MMU_FTR_KERNEL_RO		ASM_CONST(0x00004000)
+
+/*
  * We need to clear top 16bits of va (from the remaining 64 bits )in
  * tlbie* instructions
  */
···
 #define MMU_FTRS_POWER4		MMU_FTRS_DEFAULT_HPTE_ARCH_V2
 #define MMU_FTRS_PPC970		MMU_FTRS_POWER4 | MMU_FTR_TLBIE_CROP_VA
 #define MMU_FTRS_POWER5		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE
-#define MMU_FTRS_POWER6		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE
-#define MMU_FTRS_POWER7		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE
-#define MMU_FTRS_POWER8		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE
-#define MMU_FTRS_POWER9		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE
+#define MMU_FTRS_POWER6		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_KERNEL_RO
+#define MMU_FTRS_POWER7		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_KERNEL_RO
+#define MMU_FTRS_POWER8		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_KERNEL_RO
+#define MMU_FTRS_POWER9		MMU_FTRS_POWER4 | MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_KERNEL_RO
 #define MMU_FTRS_CELL		MMU_FTRS_DEFAULT_HPTE_ARCH_V2 | \
 				MMU_FTR_CI_LARGE_PAGE
 #define MMU_FTRS_PA6T		MMU_FTRS_DEFAULT_HPTE_ARCH_V2 | \

arch/powerpc/mm/hash_utils_64.c

···
 		/*
 		 * Kernel read only mapped with ppp bits 0b110
 		 */
-		if (!(pteflags & _PAGE_WRITE))
-			rflags |= (HPTE_R_PP0 | 0x2);
+		if (!(pteflags & _PAGE_WRITE)) {
+			if (mmu_has_feature(MMU_FTR_KERNEL_RO))
+				rflags |= (HPTE_R_PP0 | 0x2);
+			else
+				rflags |= 0x3;
+		}
 	} else {
 		if (pteflags & _PAGE_RWX)
 			rflags |= 0x2;