[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.lin...@arm.com>
Cc: Jonathan Corbet <cor...@lwn.net>
Cc: linux-...@vger.kernel.org
---
 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