powerpc/8xx: Copy kernel PGD entries into all PGDIRs

In order to avoid having to select PGDIR at each TLB miss based on
fault address, copy kernel PGD entries into all PGDIRs in pgd_alloc().

At first it will be used for ITLB misses for kernel TEXT, then for
execmem then for kernel DATA.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c6d2bf5af2ea909071a85bdca8b1f5dc2df134a8.1724173828.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/pgalloc.h

@@ -19,8 +19,14 @@ static inline void tlb_flush_pgtable(struct mmu_gather *tlb,
 
 static inline pgd_t *pgd_alloc(struct mm_struct *mm)
 {
-	return kmem_cache_alloc(PGT_CACHE(PGD_INDEX_SIZE),
+	pgd_t *pgd = kmem_cache_alloc(PGT_CACHE(PGD_INDEX_SIZE),
 			pgtable_gfp_flags(mm, GFP_KERNEL));
+
+#ifdef CONFIG_PPC_8xx
+	memcpy(pgd + USER_PTRS_PER_PGD, swapper_pg_dir + USER_PTRS_PER_PGD,
+	       (MAX_PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
+#endif
+	return pgd;
 }
 
 static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)