[RFC PATCH 19/56] x86/bugs: Define bugs_smt_disable()

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

bugs_smt_disable() is a wrapper around cpu_smt_disable() which can only be
called during boot. Print a warning if any mitigations try to disable SMT
at runtime.

bugs_smt_disable() is marked __ref because cpu_smt_disable() is an __init
function, but it will never be called after init code is free'd.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/kernel/cpu/bugs.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fcb1337026f1..6cd7198f7dca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -455,6 +455,14 @@ static bool __init should_mitigate_vuln(unsigned int bug)
}
}

+static void __ref bugs_smt_disable(bool enable)
+{
+ if (system_state == SYSTEM_BOOTING)
+ cpu_smt_disable(enable);
+ else
+ pr_warn("Unable to disable SMT after system boot!\n");
+}
+
/* Default mitigation for MDS-affected CPUs */
static enum mds_mitigations mds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_MDS) ? MDS_MITIGATION_AUTO : MDS_MITIGATION_OFF;
@@ -550,7 +558,7 @@ static void __init mds_apply_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
(mds_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
- cpu_smt_disable(false);
+ bugs_smt_disable(false);
}
}

@@ -671,7 +679,7 @@ static void __init taa_apply_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);

if (taa_nosmt || smt_mitigations == SMT_MITIGATIONS_ON)
- cpu_smt_disable(false);
+ bugs_smt_disable(false);
}
}

@@ -784,7 +792,7 @@ static void __init mmio_apply_mitigation(void)
static_branch_enable(&cpu_buf_idle_clear);

if (mmio_nosmt || smt_mitigations == SMT_MITIGATIONS_ON)
- cpu_smt_disable(false);
+ bugs_smt_disable(false);
}

#ifdef CONFIG_DYNAMIC_MITIGATIONS
@@ -1592,7 +1600,7 @@ static void __init retbleed_apply_mitigation(void)

if (mitigate_smt && !boot_cpu_has(X86_FEATURE_STIBP) &&
(retbleed_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
- cpu_smt_disable(false);
+ bugs_smt_disable(false);
}

#ifdef CONFIG_DYNAMIC_MITIGATIONS
@@ -3109,10 +3117,10 @@ static void __init l1tf_apply_mitigation(void)
break;
case L1TF_MITIGATION_FLUSH_NOSMT:
case L1TF_MITIGATION_FULL:
- cpu_smt_disable(false);
+ bugs_smt_disable(false);
break;
case L1TF_MITIGATION_FULL_FORCE:
- cpu_smt_disable(true);
+ bugs_smt_disable(true);
break;
}

--
2.34.1