[PATCH v9 06/10] arm64: Implement can_set_direct_map_range()

[Vincent Donnefort <vdonnefort@google.com>]

For systems where can_set_direct_map() is false, it is still possible
that a subregion of the direct map can be modified if it is mapped at
the last-level. Add an implementation for can_set_direct_map_range(). It
falls back to a page table walk to verify the mapping level if
can_set_direct_map() is false.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/arch/arm64/include/asm/set_memory.h b/arch/arm64/include/asm/set_memory.h
index 90f61b17275e..1a37f4ef130b 100644
--- a/arch/arm64/include/asm/set_memory.h
+++ b/arch/arm64/include/asm/set_memory.h
@@ -9,6 +9,9 @@
 bool can_set_direct_map(void);
 #define can_set_direct_map can_set_direct_map
 
+bool can_set_direct_map_range(struct page *page, unsigned long nr_pages);
+#define can_set_direct_map_range can_set_direct_map_range
+
 int set_memory_valid(unsigned long addr, int numpages, int enable);
 
 int set_direct_map_invalid_noflush(struct page *page);
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 58898ae53f5d..c59ef17eb0d0 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -103,6 +103,48 @@ bool can_set_direct_map(void)
 		system_supports_bbml3();
 }
 
+bool can_set_direct_map_range(struct page *page, unsigned long nr_pages)
+{
+	unsigned long addr = (unsigned long)page_address(page);
+	unsigned long end = addr + nr_pages * PAGE_SIZE;
+
+	if (can_set_direct_map())
+		return true;
+
+	/*
+	 * If !can_set_direct_map() then no one can split blocks and it is safe
+	 * to walk the page-table lockless.
+	 */
+	while (addr < end) {
+		pud_t *pudp, pud;
+		pmd_t *pmdp, pmd;
+		pgd_t *pgdp;
+		p4d_t *p4dp;
+
+		pgdp = pgd_offset_k(addr);
+		if (pgd_none(READ_ONCE(*pgdp)))
+			return false;
+
+		p4dp = p4d_offset(pgdp, addr);
+		if (p4d_none(READ_ONCE(*p4dp)))
+			return false;
+
+		pudp = pud_offset(p4dp, addr);
+		pud = READ_ONCE(*pudp);
+		if (pud_none(pud) || pud_leaf(pud))
+			return false;
+
+		pmdp = pmd_offset(pudp, addr);
+		pmd = READ_ONCE(*pmdp);
+		if (pmd_none(pmd) || pmd_leaf(pmd))
+			return false;
+
+		addr = pmd_addr_end(addr, end);
+	}
+
+	return true;
+}
+
 static int update_range_prot(unsigned long start, unsigned long size,
 			     pgprot_t set_mask, pgprot_t clear_mask)
 {
-- 
2.55.0.970.g62bdec98f9-goog