arm64: mm: Ensure writes to swapper are ordered wrt subsequent cache maintenance

When rewriting swapper using nG mappings, we must performance cache
maintenance around each page table access in order to avoid coherency
problems with the host's cacheable alias under KVM. To ensure correct
ordering of the maintenance with respect to Device memory accesses made
with the Stage-1 MMU disabled, DMBs need to be added between the
maintenance and the corresponding memory access.

This patch adds a missing DMB between writing a new page table entry and
performing a clean+invalidate on the same line.

Fixes: f992b4df ("arm64: kpti: Add ->enable callback to remap swapper using nG mappings")
Cc: <stable@vger.kernel.org> # 4.16.x-
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/proc.S

@@ -217,8 +217,9 @@ ENDPROC(idmap_cpu_replace_ttbr1)
 
 	.macro __idmap_kpti_put_pgtable_ent_ng, type
 	orr	\type, \type, #PTE_NG		// Same bit for blocks and pages
-	str	\type, [cur_\()\type\()p]	// Update the entry and ensure it
-	dc	civac, cur_\()\type\()p		// is visible to all CPUs.
+	str	\type, [cur_\()\type\()p]	// Update the entry and ensure
+	dmb	sy				// that it is visible to all
+	dc	civac, cur_\()\type\()p		// CPUs.
 	.endm
 
 /*