[RFC/RFT PATCH 09/16] arm64: Provide a command line to disable spectre_v2 mitigation

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: linux-arm-kernel@lists.infradead.org
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Jeremy Linton <jeremy.linton@arm.com>,
	Andre Przywara <andre.przywara@arm.com>,
	Stefan Wahren <stefan.wahren@i2se.com>,
	Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org, Will Deacon <will.deacon@arm.com>
Subject: [RFC/RFT PATCH 09/16] arm64: Provide a command line to disable spectre_v2 mitigation
Date: Fri,  4 Oct 2019 14:04:23 +0200	[thread overview]
Message-ID: <20191004120430.11929-10-ard.biesheuvel@linaro.org> (raw)
In-Reply-To: <20191004120430.11929-1-ard.biesheuvel@linaro.org>

From: Jeremy Linton <jeremy.linton@arm.com>

There are various reasons, such as benchmarking, to disable spectrev2
mitigation on a machine. Provide a command-line option to do so.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit e5ce5e7267ddcbe13ab9ead2542524e1b7993e5a)
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.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 e8ddf0ef232e..cc2f5c9a8161 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2866,10 +2866,10 @@
 			(bounds 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 3758ba538a43..5a7fa90c668f 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -189,6 +189,14 @@ 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)
 {
@@ -200,6 +208,11 @@ 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

