[RFC PATCH 08/56] x86/bugs: Reset SSB mitigations

David Kaplan
2025-10-13T14:35:28Z

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

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/kernel/cpu/bugs.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4ca46f58e384..cc7b1b67d22d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -380,6 +380,16 @@ static void x86_amd_ssb_disable(void)
wrmsrq(MSR_AMD64_LS_CFG, msrval);
}

+static void x86_amd_ssb_enable(void)
+{
+ u64 msrval = x86_amd_ls_cfg_base;
+
+ if (boot_cpu_has(X86_FEATURE_VIRT_SSBD))
+ wrmsrl(MSR_AMD64_VIRT_SPEC_CTRL, 0);
+ else if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD))
+ wrmsrl(MSR_AMD64_LS_CFG, msrval);
+}
+
#undef pr_fmt
#define pr_fmt(fmt) "MDS: " fmt

@@ -2672,6 +2682,17 @@ static void __init ssb_apply_mitigation(void)
}
}

+#ifdef CONFIG_DYNAMIC_MITIGATIONS
+static void ssb_reset_mitigation(void)
+{
+ setup_clear_cpu_cap(X86_FEATURE_SPEC_STORE_BYPASS_DISABLE);
+ x86_spec_ctrl_base &= ~SPEC_CTRL_SSBD;
+ nossb = false;
+ ssb_mode = IS_ENABLED(CONFIG_MITIGATION_SSB) ?
+ SPEC_STORE_BYPASS_AUTO : SPEC_STORE_BYPASS_NONE;
+}
+#endif
+
#undef pr_fmt
#define pr_fmt(fmt) "Speculation prctl: " fmt

@@ -2916,6 +2937,8 @@ void x86_spec_ctrl_setup_ap(void)

if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
x86_amd_ssb_disable();
+ else
+ x86_amd_ssb_enable();
}

bool itlb_multihit_kvm_mitigation;
@@ -3857,5 +3880,6 @@ void arch_cpu_reset_mitigations(void)
spectre_v2_reset_mitigation();
retbleed_reset_mitigation();
spectre_v2_user_reset_mitigation();
+ ssb_reset_mitigation();
}
#endif
--
2.34.1