[PATCH v2 1/2] arm64: capabilities: add nopti command line argument

[mlangsdo@redhat.com (Mark Langsdorf)]

The x86 kernel and the documentation use 'nopti' as the kernel command
line argument to disable kernel page table isolation, so add nopti to
the arm64 kernel for compatibility.

Signed-off-by: Mark Langsdorf <mlangsdo@redhat.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 19 +++++++++++++++----
 arch/arm64/kernel/cpufeature.c                  | 11 ++++++++++-
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2040d4..cf0c728 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3342,8 +3342,8 @@
 	pt.		[PARIDE]
 			See Documentation/blockdev/paride.txt.
 
-	pti=		[X86_64] Control Page Table Isolation of user and
-			kernel address spaces.  Disabling this feature
+	pti=		[X86_64] Control Page Table Isolation of user
+			and kernel address spaces.  Disabling this feature
 			removes hardening, but improves performance of
 			system calls and interrupts.
 
@@ -3354,8 +3354,19 @@
 
 			Not specifying this option is equivalent to pti=auto.
 
-	nopti		[X86_64]
-			Equivalent to pti=off
+        kpti=           [ARM64] Control Page Table Isolation of user
+                        and kernel address spaces.  Disabling this feature
+                        removes hardening, but improves performance of
+                        system calls and interrupts.
+
+                        on   - unconditionally enable
+                        off  - unconditionally disable
+
+                        Not specifying this option will enable kpti on all
+			systems that support it.
+
+	nopti		[X86_64, ARM64]
+			Equivalent to pti=off on X86_64 or kpti=off on ARM64
 
 	pty.legacy_count=
 			[KNL] Number of legacy pty's. Overwrites compiled-in
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9d1b06d..7c5d8712 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -934,10 +934,19 @@ static int __init parse_kpti(char *str)
 	if (ret)
 		return ret;
 
-	__kpti_forced = enabled ? 1 : -1;
+	if (!__kpti_forced)
+		__kpti_forced = enabled ? 1 : -1;
 	return 0;
 }
 __setup("kpti=", parse_kpti);
+
+/* for compatibility with documentation and x86 nopti command line arg */
+static int __init force_nokpti(char *arg)
+{
+	__kpti_forced = -1;
+	return 0;
+}
+early_param("nopti", force_nokpti);
 #endif	/* CONFIG_UNMAP_KERNEL_AT_EL0 */
 
 #ifdef CONFIG_ARM64_HW_AFDBM
-- 
2.9.5