[PATCH V5 1/2] arm64: refactor the rodata=xxx

[Huang Shijie <shijie@os.amperecomputing.com>]

As per admin guide documentation, "rodata=on" should be the default on
platforms. Documentation/admin-guide/kernel-parameters.txt describes
these options as

   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
                   [arm64]

But on arm64 platform, "rodata=full" is the default instead. This patch
implements the following changes.

 - Make "rodata=on" behaviour same as the original "rodata=full"
 - Make "rodata=noalias" (new) behaviour same as the original "rodata=on"
 - Drop the original "rodata=full"
 - Add comment for arch_parse_debug_rodata()
 - Update kernel-parameters.txt as required

After this patch, the "rodata=on" will be the default on arm64 platform
as well.

Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
---
 .../admin-guide/kernel-parameters.txt         |  2 +-
 arch/arm64/include/asm/setup.h                | 28 +++++++++++++++++--
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ee0735c6b8e2..e0cd6dac26d3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6354,7 +6354,7 @@
 	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..6b994d0881d1 100644
--- a/arch/arm64/include/asm/setup.h
+++ b/arch/arm64/include/asm/setup.h
@@ -13,6 +13,30 @@
 extern phys_addr_t __fdt_pointer __initdata;
 extern u64 __cacheline_aligned boot_args[4];
 
+/*
+ * rodata=on (default)
+ *
+ *    This applies read-only attributes to VM areas and 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 for the same memory page.
+ *
+ *    But this might cause linear map region to be mapped down to base
+ *    pages, which may adversely affect performance in some cases.
+ *
+ * rodata=off
+ *
+ *    This provides more block mappings and contiguous hints for linear
+ *    map region which would minimize TLB footprint. This also leaves
+ *    read-only kernel memory writable for debugging.
+ *
+ * rodata=noalias
+ *
+ *    This provides more block mappings and contiguous hints for linear
+ *    map region which would minimize TLB footprint. This leaves the linear
+ *    alias of read-only mappings in the vmalloc space writeable, making
+ *    them susceptible to inadvertent modification by software.
+ */
 static inline bool arch_parse_debug_rodata(char *arg)
 {
 	extern bool rodata_enabled;
@@ -21,7 +45,7 @@ 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 +55,7 @@ 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