[PATCH v5 01/16] x86/bugs: Restructure MDS mitigation

David Kaplan
2025-04-18T16:17:38Z

Restructure MDS mitigation selection to use select/update/apply
functions to create consistent vulnerability handling.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/kernel/cpu/bugs.c | 58 ++++++++++++++++++++++++++++++++++++--
1 file changed, 56 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9387f5f9de12..4295502ea082 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -34,6 +34,25 @@

#include "cpu.h"

+/*
+ * Speculation Vulnerability Handling
+ *
+ * Each vulnerability is handled with the following functions:
+ * <vuln>_select_mitigation() -- Selects a mitigation to use. This should
+ * take into account all relevant command line
+ * options.
+ * <vuln>_update_mitigation() -- This is called after all vulnerabilities have
+ * selected a mitigation, in case the selection
+ * may want to change based on other choices
+ * made. This function is optional.
+ * <vuln>_apply_mitigation() -- Enable the selected mitigation.
+ *
+ * The compile-time mitigation in all cases should be AUTO. An explicit
+ * command-line option can override AUTO. If no such option is
+ * provided, <vuln>_select_mitigation() will override AUTO to the best
+ * mitigation option.
+ */
+
static void __init spectre_v1_select_mitigation(void);
static void __init spectre_v2_select_mitigation(void);
static void __init retbleed_select_mitigation(void);
@@ -41,6 +60,8 @@ static void __init spectre_v2_user_select_mitigation(void);
static void __init ssb_select_mitigation(void);
static void __init l1tf_select_mitigation(void);
static void __init mds_select_mitigation(void);
+static void __init mds_update_mitigation(void);
+static void __init mds_apply_mitigation(void);
static void __init md_clear_update_mitigation(void);
static void __init md_clear_select_mitigation(void);
static void __init taa_select_mitigation(void);
@@ -172,6 +193,7 @@ void __init cpu_select_mitigations(void)
spectre_v2_user_select_mitigation();
ssb_select_mitigation();
l1tf_select_mitigation();
+ mds_select_mitigation();
md_clear_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
@@ -182,6 +204,14 @@ void __init cpu_select_mitigations(void)
*/
srso_select_mitigation();
gds_select_mitigation();
+
+ /*
+ * After mitigations are selected, some may need to update their
+ * choices.
+ */
+ mds_update_mitigation();
+
+ mds_apply_mitigation();
}

/*
@@ -284,6 +314,9 @@ enum rfds_mitigations {
static enum rfds_mitigations rfds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_AUTO : RFDS_MITIGATION_OFF;

+/* Set if any of MDS/TAA/MMIO/RFDS are going to enable VERW. */
+static bool verw_mitigation_selected __ro_after_init;
+
static void __init mds_select_mitigation(void)
{
if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
@@ -294,12 +327,34 @@ static void __init mds_select_mitigation(void)
if (mds_mitigation == MDS_MITIGATION_AUTO)
mds_mitigation = MDS_MITIGATION_FULL;

+ if (mds_mitigation == MDS_MITIGATION_OFF)
+ return;
+
+ verw_mitigation_selected = true;
+}
+
+static void __init mds_update_mitigation(void)
+{
+ if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off())
+ return;
+
+ /* If TAA, MMIO, or RFDS are being mitigated, MDS gets mitigated too. */
+ if (verw_mitigation_selected)
+ mds_mitigation = MDS_MITIGATION_FULL;
+
if (mds_mitigation == MDS_MITIGATION_FULL) {
if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
mds_mitigation = MDS_MITIGATION_VMWERV;
+ }

- setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ pr_info("%s\n", mds_strings[mds_mitigation]);
+}

+static void __init mds_apply_mitigation(void)
+{
+ if (mds_mitigation == MDS_MITIGATION_FULL ||
+ mds_mitigation == MDS_MITIGATION_VMWERV) {
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
(mds_nosmt || cpu_mitigations_auto_nosmt()))
cpu_smt_disable(false);
@@ -599,7 +654,6 @@ static void __init md_clear_update_mitigation(void)

static void __init md_clear_select_mitigation(void)
{
- mds_select_mitigation();
taa_select_mitigation();
mmio_select_mitigation();
rfds_select_mitigation();
--
2.34.1