[PATCH v5 02/16] x86/bugs: Restructure TAA mitigation

David Kaplan
2025-04-18T16:17:37Z

Restructure TAA mitigation 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 | 94 ++++++++++++++++++++++++--------------
1 file changed, 59 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4295502ea082..c0ba034ae1f9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -65,6 +65,8 @@ 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);
+static void __init taa_update_mitigation(void);
+static void __init taa_apply_mitigation(void);
static void __init mmio_select_mitigation(void);
static void __init srbds_select_mitigation(void);
static void __init l1d_flush_select_mitigation(void);
@@ -194,6 +196,7 @@ void __init cpu_select_mitigations(void)
ssb_select_mitigation();
l1tf_select_mitigation();
mds_select_mitigation();
+ taa_select_mitigation();
md_clear_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
@@ -210,8 +213,10 @@ void __init cpu_select_mitigations(void)
* choices.
*/
mds_update_mitigation();
+ taa_update_mitigation();

mds_apply_mitigation();
+ taa_apply_mitigation();
}

/*
@@ -394,6 +399,11 @@ static const char * const taa_strings[] = {
[TAA_MITIGATION_TSX_DISABLED] = "Mitigation: TSX disabled",
};

+static bool __init taa_vulnerable(void)
+{
+ return boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM);
+}
+
static void __init taa_select_mitigation(void)
{
if (!boot_cpu_has_bug(X86_BUG_TAA)) {
@@ -407,48 +417,63 @@ static void __init taa_select_mitigation(void)
return;
}

- if (cpu_mitigations_off()) {
+ if (cpu_mitigations_off())
taa_mitigation = TAA_MITIGATION_OFF;
- return;
- }

- /*
- * TAA mitigation via VERW is turned off if both
- * tsx_async_abort=off and mds=off are specified.
- */
- if (taa_mitigation == TAA_MITIGATION_OFF &&
- mds_mitigation == MDS_MITIGATION_OFF)
+ /* Microcode will be checked in taa_update_mitigation(). */
+ if (taa_mitigation == TAA_MITIGATION_AUTO)
+ taa_mitigation = TAA_MITIGATION_VERW;
+
+ if (taa_mitigation != TAA_MITIGATION_OFF)
+ verw_mitigation_selected = true;
+}
+
+static void __init taa_update_mitigation(void)
+{
+ if (!taa_vulnerable() || cpu_mitigations_off())
return;

- if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
+ if (verw_mitigation_selected)
taa_mitigation = TAA_MITIGATION_VERW;
- else
- taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;

- /*
- * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
- * A microcode update fixes this behavior to clear CPU buffers. It also
- * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
- * ARCH_CAP_TSX_CTRL_MSR bit.
- *
- * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
- * update is required.
- */
- if ( (x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
- !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
- taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+ if (taa_mitigation == TAA_MITIGATION_VERW) {
+ /* Check if the requisite ucode is available. */
+ if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
+ taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;

- /*
- * TSX is enabled, select alternate mitigation for TAA which is
- * the same as MDS. Enable MDS static branch to clear CPU buffers.
- *
- * For guests that can't determine whether the correct microcode is
- * present on host, enable the mitigation for UCODE_NEEDED as well.
- */
- setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+ /*
+ * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
+ * A microcode update fixes this behavior to clear CPU buffers. It also
+ * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
+ * ARCH_CAP_TSX_CTRL_MSR bit.
+ *
+ * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
+ * update is required.
+ */
+ if ((x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
+ !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
+ taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+ }

- if (taa_nosmt || cpu_mitigations_auto_nosmt())
- cpu_smt_disable(false);
+ pr_info("%s\n", taa_strings[taa_mitigation]);
+}
+
+static void __init taa_apply_mitigation(void)
+{
+ if (taa_mitigation == TAA_MITIGATION_VERW ||
+ taa_mitigation == TAA_MITIGATION_UCODE_NEEDED) {
+ /*
+ * TSX is enabled, select alternate mitigation for TAA which is
+ * the same as MDS. Enable MDS static branch to clear CPU buffers.
+ *
+ * For guests that can't determine whether the correct microcode is
+ * present on host, enable the mitigation for UCODE_NEEDED as well.
+ */
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+ if (taa_nosmt || cpu_mitigations_auto_nosmt())
+ cpu_smt_disable(false);
+ }
}

static int __init tsx_async_abort_parse_cmdline(char *str)
@@ -654,7 +679,6 @@ static void __init md_clear_update_mitigation(void)

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

--
2.34.1