Reset global mitigations, including attack vectors and then call the
arch-specific function to reset further mitigations.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
 include/linux/cpu.h |  3 +++
 kernel/cpu.c        | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 487b3bf2e1ea..3da629a76a49 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -208,6 +208,9 @@ enum smt_mitigations {
 	SMT_MITIGATIONS_ON,
 };
 
+void cpu_reset_mitigations(void);
+void arch_cpu_reset_mitigations(void);
+
 #ifdef CONFIG_CPU_MITIGATIONS
 extern bool cpu_mitigations_off(void);
 extern bool cpu_mitigations_auto_nosmt(void);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index db9f6c539b28..910249f6217a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3171,6 +3171,7 @@ void __init boot_cpu_hotplug_init(void)
 }
 
 #ifdef CONFIG_CPU_MITIGATIONS
+
 /*
  * All except the cross-thread attack vector are mitigated by default.
  * Cross-thread mitigation often requires disabling SMT which is expensive
@@ -3326,3 +3327,20 @@ static int __init mitigations_parse_cmdline(char *arg)
 }
 #endif
 early_param("mitigations", mitigations_parse_cmdline);
+
+#ifdef CONFIG_DYNAMIC_MITIGATIONS
+void __weak arch_cpu_reset_mitigations(void)
+{
+}
+
+void cpu_reset_mitigations(void)
+{
+	smt_mitigations = SMT_MITIGATIONS_AUTO;
+	cpu_mitigations = CPU_MITIGATIONS_AUTO;
+	attack_vectors[CPU_MITIGATE_USER_KERNEL] = true;
+	attack_vectors[CPU_MITIGATE_USER_USER] = true;
+	attack_vectors[CPU_MITIGATE_GUEST_HOST] = IS_ENABLED(CONFIG_KVM);
+	attack_vectors[CPU_MITIGATE_GUEST_GUEST] = IS_ENABLED(CONFIG_KVM);
+	arch_cpu_reset_mitigations();
+}
+#endif
-- 
2.34.1

On Mon, Oct 13, 2025 at 09:33:51AM -0500, David Kaplan wrote:
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +void __weak arch_cpu_reset_mitigations(void)
> +{
> +}
> +
> +void cpu_reset_mitigations(void)
> +{
> +	smt_mitigations = SMT_MITIGATIONS_AUTO;
> +	cpu_mitigations = CPU_MITIGATIONS_AUTO;
> +	attack_vectors[CPU_MITIGATE_USER_KERNEL] = true;
> +	attack_vectors[CPU_MITIGATE_USER_USER] = true;
> +	attack_vectors[CPU_MITIGATE_GUEST_HOST] = IS_ENABLED(CONFIG_KVM);
> +	attack_vectors[CPU_MITIGATE_GUEST_GUEST] = IS_ENABLED(CONFIG_KVM);
> +	arch_cpu_reset_mitigations();
> +}
> +#endif

Considering this will have no effect on other arches (or even on x86 at
this point in the series), should CONFIG_DYNAMIC_MITIGATIONS depend on
an arch-specific CONFIG_HAVE_DYNAMIC_MITIGATIONS?

Then the weak function can be removed (and weak functions should be
avoided anyway, IMO).

-- 
Josh