arm64: head: add helper function to remap regions in early page tables

The asm macros used to create the initial ID map and kernel mappings
don't support randomly remapping parts of the address space after it has
been populated. What we can do, however, given that all block or page
mappings are created at the final level, is take a subset of the mapped
range and update its attributes or output address. This will permit us
to make parts of these page tables read-only, or remap a part of it to
cover the device tree.

So add a helper that encapsulates this.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220624150651.1358849-12-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/head.S

@@ -263,6 +263,39 @@ SYM_FUNC_END(clear_page_tables)
 	populate_entries \tbl, \rtbl, \istart, \iend, \flags, #SWAPPER_BLOCK_SIZE, \tmp
 	.endm
 
+/*
+ * Remap a subregion created with the map_memory macro with modified attributes
+ * or output address. The entire remapped region must have been covered in the
+ * invocation of map_memory.
+ *
+ * x0: last level table address (returned in first argument to map_memory)
+ * x1: start VA of the existing mapping
+ * x2: start VA of the region to update
+ * x3: end VA of the region to update (exclusive)
+ * x4: start PA associated with the region to update
+ * x5: attributes to set on the updated region
+ * x6: order of the last level mappings
+ */
+SYM_FUNC_START_LOCAL(remap_region)
+	sub	x3, x3, #1		// make end inclusive
+
+	// Get the index offset for the start of the last level table
+	lsr	x1, x1, x6
+	bfi	x1, xzr, #0, #PAGE_SHIFT - 3
+
+	// Derive the start and end indexes into the last level table
+	// associated with the provided region
+	lsr	x2, x2, x6
+	lsr	x3, x3, x6
+	sub	x2, x2, x1
+	sub	x3, x3, x1
+
+	mov	x1, #1
+	lsl	x6, x1, x6		// block size at this level
+
+	populate_entries x0, x4, x2, x3, x5, x6, x7
+	ret
+SYM_FUNC_END(remap_region)
 
 SYM_FUNC_START_LOCAL(create_idmap)
 	adrp	x0, idmap_pg_dir