[PATCH v5 01/10] arm64: Provide a command line to disable spectre_v2 mitigation

[Jeremy Linton]

There are various reasons, including bencmarking, to disable spectrev2
mitigation on a machine. Provide a command-line to do so.

Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
---
 Documentation/admin-guide/kernel-parameters.txt |  8 ++++----
 arch/arm64/kernel/cpu_errata.c                  | 13 +++++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 858b6c0b9a15..4d4d6a9537ae 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2842,10 +2842,10 @@
 			check bypass). With this option data leaks are possible
 			in the system.
 
-	nospectre_v2	[X86,PPC_FSL_BOOK3E] Disable all mitigations for the Spectre variant 2
-			(indirect branch prediction) vulnerability. System may
-			allow data leaks with this option, which is equivalent
-			to spectre_v2=off.
+	nospectre_v2	[X86,PPC_FSL_BOOK3E,ARM64] Disable all mitigations for
+			the Spectre variant 2 (indirect branch prediction)
+			vulnerability. System may allow data leaks with this
+			option.
 
 	nospec_store_bypass_disable
 			[HW] Disable all mitigations for the Speculative Store Bypass vulnerability
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 9950bb0cbd52..d2b2c69d31bb 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -220,6 +220,14 @@ static void qcom_link_stack_sanitization(void)
 		     : "=&r" (tmp));
 }
 
+static bool __nospectre_v2;
+static int __init parse_nospectre_v2(char *str)
+{
+	__nospectre_v2 = true;
+	return 0;
+}
+early_param("nospectre_v2", parse_nospectre_v2);
+
 static void
 enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
 {
@@ -231,6 +239,11 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
 	if (!entry->matches(entry, SCOPE_LOCAL_CPU))
 		return;
 
+	if (__nospectre_v2) {
+		pr_info_once("spectrev2 mitigation disabled by command line option\n");
+		return;
+	}
+
 	if (psci_ops.smccc_version == SMCCC_VERSION_1_0)
 		return;
 
-- 
2.20.1