x86/bugs: Restructure TAA mitigation

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

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-3-david.kaplan@amd.com

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index f697e6b41afd3496b3f201a4815872cc0413e337..5db21d2ee87dd175f1f45ec5b68ae89ab4418a35 100644 (file)
--- 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();
 }
 
 /*
@@ -397,6 +402,11 @@ 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)) {
@@ -410,48 +420,63 @@ 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_clear_cpu_buf_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_clear_cpu_buf_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)
@@ -660,7 +685,6 @@ out:
 
 static void __init md_clear_select_mitigation(void)
 {
-       taa_select_mitigation();
        mmio_select_mitigation();
        rfds_select_mitigation();