Add function to reset spectre_v2 mitigations back to their boot-time
defaults.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
 arch/x86/kernel/cpu/bugs.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9d5c6a3e50e1..0430635bb17d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2477,6 +2477,24 @@ static void __init spectre_v2_apply_mitigation(void)
 	}
 }
 
+#ifdef CONFIG_DYNAMIC_MITIGATIONS
+static void spectre_v2_reset_mitigation(void)
+{
+	x86_spec_ctrl_base &= ~SPEC_CTRL_IBRS;
+	x86_spec_ctrl_base &= ~SPEC_CTRL_RRSBA_DIS_S;
+	rrsba_disabled = false;
+	setup_clear_cpu_cap(X86_FEATURE_KERNEL_IBRS);
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE_LFENCE);
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE);
+	setup_clear_cpu_cap(X86_FEATURE_RSB_CTXSW);
+	setup_clear_cpu_cap(X86_FEATURE_USE_IBPB_FW);
+	spectre_v2_enabled = SPECTRE_V2_NONE;
+	nospectre_v2 = false;
+	spectre_v2_cmd = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?
+		SPECTRE_V2_CMD_AUTO : SPECTRE_V2_CMD_NONE;
+}
+#endif
+
 static void update_stibp_msr(void * __unused)
 {
 	u64 val = spec_ctrl_current() | (x86_spec_ctrl_base & SPEC_CTRL_STIBP);
@@ -3810,5 +3828,6 @@ void __warn_thunk(void)
 void arch_cpu_reset_mitigations(void)
 {
 	spectre_v1_reset_mitigation();
+	spectre_v2_reset_mitigation();
 }
 #endif
-- 
2.34.1

On Mon, Oct 13, 2025 at 09:33:53AM -0500, David Kaplan wrote:
> Add function to reset spectre_v2 mitigations back to their boot-time
> defaults.
> 
> Signed-off-by: David Kaplan <david.kaplan@amd.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 9d5c6a3e50e1..0430635bb17d 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2477,6 +2477,24 @@ static void __init spectre_v2_apply_mitigation(void)
>  	}
>  }
>  
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void spectre_v2_reset_mitigation(void)

Yeah, the reset should definitely be a single function - this ifdeffery around
every single one is going to become yucky.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette