[PATCH v2] arm64: spectre: increase parameters that can be used to turn off bhb mitigation individually

[Liu Song]

From: Liu Song <liusong@xxxxxxxxxxxxxxxxx>

In our environment, it was found that the mitigation BHB has a great
impact on the benchmark performance. For example, in the lmbench test,
the "process fork && exit" test performance drops by 20%.
So it is necessary to have the ability to turn off the mitigation
individually through cmdline, thus avoiding having to compile the
kernel by adjusting the config.

Signed-off-by: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  5 +++++
 arch/arm64/kernel/proton-pack.c                 | 10 +++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d7f3090..1edc9a6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3207,6 +3207,7 @@
 					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]
 					       ssbd=force-off [ARM64]
+					       nospectre_bhb [ARM64]
 					       l1tf=off [X86]
 					       mds=off [X86]
 					       tsx_async_abort=off [X86]
@@ -3631,6 +3632,10 @@
 			vulnerability. System may allow data leaks with this
 			option.
 
+	nospectre_bhb	[ARM64] Disable all mitigations for Spectre-BHB (branch
+			history injection) 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/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index 40be3a7..bd16903 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -988,6 +988,14 @@ static void this_cpu_set_vectors(enum arm64_bp_harden_el1_vectors slot)
 	isb();
 }
 
+static bool __read_mostly __nospectre_bhb;
+static int __init parse_spectre_bhb_param(char *str)
+{
+	__nospectre_bhb = true;
+	return 0;
+}
+early_param("nospectre_bhb", parse_spectre_bhb_param);
+
 void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 {
 	bp_hardening_cb_t cpu_cb;
@@ -1001,7 +1009,7 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 		/* No point mitigating Spectre-BHB alone. */
 	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
 		pr_info_once("spectre-bhb mitigation disabled by compile time option\n");
-	} else if (cpu_mitigations_off()) {
+	} else if (cpu_mitigations_off() || __nospectre_bhb) {
 		pr_info_once("spectre-bhb mitigation disabled by command line option\n");
 	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
 		state = SPECTRE_MITIGATED;
-- 
1.8.3.1