x86/speculation/mds: Add SMT warning message

MDS is vulnerable with SMT.  Make that clear with a one-time printk
whenever SMT first gets enabled.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>

arch/x86/kernel/cpu/bugs.c

@@ -673,6 +673,9 @@ static void update_indir_branch_cond(void)
 		static_branch_disable(&switch_to_cond_stibp);
 }
 
+#undef pr_fmt
+#define pr_fmt(fmt) fmt
+
 /* Update the static key controlling the MDS CPU buffer clear in idle */
 static void update_mds_branch_idle(void)
 {
@@ -693,6 +696,8 @@ static void update_mds_branch_idle(void)
 		static_branch_disable(&mds_idle_clear);
 }
 
+#define MDS_MSG_SMT "MDS CPU bug present and SMT on, data leak possible. See https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html for more details.\n"
+
 void arch_smt_update(void)
 {
 	/* Enhanced IBRS implies STIBP. No update required. */
@@ -717,6 +722,8 @@ void arch_smt_update(void)
 	switch (mds_mitigation) {
 	case MDS_MITIGATION_FULL:
 	case MDS_MITIGATION_VMWERV:
+		if (sched_smt_active() && !boot_cpu_has(X86_BUG_MSBDS_ONLY))
+			pr_warn_once(MDS_MSG_SMT);
 		update_mds_branch_idle();
 		break;
 	case MDS_MITIGATION_OFF:
@@ -1149,6 +1156,7 @@ static int __init l1tf_cmdline(char *str)
 early_param("l1tf", l1tf_cmdline);
 
 #undef pr_fmt
+#define pr_fmt(fmt) fmt
 
 #ifdef CONFIG_SYSFS