arm64: head: avoid cache invalidation when entering with the MMU on

If we enter with the MMU on, there is no need for explicit cache
invalidation for stores to memory, as they will be coherent with the
caches.

Let's take advantage of this, and create the ID map with the MMU still
enabled if that is how we entered, and avoid any cache invalidation
calls in that case.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230111102236.1430401-5-ardb@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/head.S

@@ -89,9 +89,9 @@
 SYM_CODE_START(primary_entry)
 	bl	record_mmu_state
 	bl	preserve_boot_args
+	bl	create_idmap
 	bl	init_kernel_el			// w0=cpu_boot_mode
 	mov	x20, x0
-	bl	create_idmap
 
 	/*
 	 * The following calls CPU setup code, see arch/arm64/mm/proc.S for
@@ -377,12 +377,13 @@ SYM_FUNC_START_LOCAL(create_idmap)
 	 * accesses (MMU disabled), invalidate those tables again to
 	 * remove any speculatively loaded cache lines.
 	 */
+	cbnz	x19, 0f				// skip cache invalidation if MMU is on
 	dmb	sy
 
 	adrp	x0, init_idmap_pg_dir
 	adrp	x1, init_idmap_pg_end
 	bl	dcache_inval_poc
-	ret	x28
+0:	ret	x28
 SYM_FUNC_END(create_idmap)
 
 SYM_FUNC_START_LOCAL(create_kernel_mapping)