[PATCH v3 1/2] arm64: refactor the rodata=xxx

[Huang Shijie]

>From Documentation/admin-guide/kernel-parameters.txt:
	rodata=		[KNL,EARLY]
		on	Mark read-only kernel memory as read-only (default).

So "rodata=on" is the default.

But the current code does not follow the document, it makes "rodata=full"
as the default.

This patch refactors the "rodata=xxx" :
      1.) Make rodata=on behaviour be the original rodata=full.
          (Drop the original rodata=full.)
      2.) Introduce "rodata=noalias" which is the original "rodata=on"
      3.) Add more comment for arch_parse_debug_rodata(),
          update kernel-parameters.txt.

After this patch, the rodata=on will be the default.

Signed-off-by: Huang Shijie <shijie@xxxxxxxxxxxxxxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  2 +-
 arch/arm64/include/asm/setup.h                | 25 +++++++++++++++++--
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a22b7e621007..51bce7b9d805 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5901,7 +5901,7 @@
 	rodata=		[KNL,EARLY]
 		on	Mark read-only kernel memory as read-only (default).
 		off	Leave read-only kernel memory writable for debugging.
-		full	Mark read-only kernel memory and aliases as read-only
+		noalias	Use more block mappings,may have better performance.
 		        [arm64]
 
 	rockchip.usb_uart
diff --git a/arch/arm64/include/asm/setup.h b/arch/arm64/include/asm/setup.h
index ba269a7a3201..b3063326b79a 100644
--- a/arch/arm64/include/asm/setup.h
+++ b/arch/arm64/include/asm/setup.h
@@ -13,6 +13,27 @@
 extern phys_addr_t __fdt_pointer __initdata;
 extern u64 __cacheline_aligned boot_args[4];
 
+/*
+ * rodata=on (default):
+ *    Apply read-only attributes of VM areas to the linear alias of
+ *    the backing pages as well. This prevents code or read-only data
+ *    from being modified (inadvertently or intentionally) via another
+ *    mapping of the same memory page.
+ *
+ *    This requires the linear region to be mapped down to pages,
+ *    which may adversely affect performance in some cases.
+ *
+ * rodata=off:
+ *    It provides us more block mappings and contiguous hits
+ *    to map the linear region which minimize the TLB footprint.
+ *    Leave read-only kernel memory writable for debugging.
+ *
+ * rodata=noalias:
+ *    It provides us more block mappings and contiguous hits
+ *    to map the linear region which minimize the TLB footprint.
+ *    And the linear aliases of pages belonging to read-only mappings
+ *    in vmalloc region are also marked as read-only.
+ */
 static inline bool arch_parse_debug_rodata(char *arg)
 {
 	extern bool rodata_enabled;
@@ -21,7 +42,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
 	if (!arg)
 		return false;
 
-	if (!strcmp(arg, "full")) {
+	if (!strcmp(arg, "on")) {
 		rodata_enabled = rodata_full = true;
 		return true;
 	}
@@ -31,7 +52,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
 		return true;
 	}
 
-	if (!strcmp(arg, "on")) {
+	if (!strcmp(arg, "noalias")) {
 		rodata_enabled = true;
 		rodata_full = false;
 		return true;
-- 
2.40.1