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

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Hi Jeremy

On 27/02/2019 01:05, Jeremy Linton wrote:
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


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;
+	}
+

Could we not disable the "cap" altogether instead, rather than disabling the
work around ? Or do we need that information ?

Cheers
Suzuki



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