arm64: head.S: ensure idmap_t0sz is visible

We write idmap_t0sz with SCTLR_EL1.{C,M} clear, but we only have the
guarnatee that the kernel Image is clean, not invalid in the caches, and
therefore we might read a stale value once the MMU is enabled.

This patch ensures we invalidate the corresponding cacheline after the
write as we do for all other data written before we set SCTLR_EL1.{C.M},
guaranteeing that the value will be visible later. We rely on the DSBs
in __create_page_tables to complete the maintenance.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/head.S

@@ -414,7 +414,10 @@ __create_page_tables:
 	cmp	x5, TCR_T0SZ(VA_BITS)	// default T0SZ small enough?
 	b.ge	1f			// .. then skip additional level
 
-	str_l	x5, idmap_t0sz, x6
+	adr_l	x6, idmap_t0sz
+	str	x5, [x6]
+	dmb	sy
+	dc	ivac, x6		// Invalidate potentially stale cache line
 
 	create_table_entry x0, x3, EXTRA_SHIFT, EXTRA_PTRS, x5, x6
 1: