x86/speculation/mds: Add mitigation mode VMWERV

In virtualized environments it can happen that the host has the microcode
update which utilizes the VERW instruction to clear CPU buffers, but the
hypervisor is not yet updated to expose the X86_FEATURE_MD_CLEAR CPUID bit
to guests.

Introduce an internal mitigation mode VMWERV which enables the invocation
of the CPU buffer clearing even if X86_FEATURE_MD_CLEAR is not set. If the
system has no updated microcode this results in a pointless execution of
the VERW instruction wasting a few CPU cycles. If the microcode is updated,
but not exposed to a guest then the CPU buffers will be cleared.

That said: Virtual Machines Will Eventually Receive Vaccine

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>

arch/x86/kernel/cpu/bugs.c

@@ -224,7 +224,8 @@ static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL
 
 static const char * const mds_strings[] = {
 	[MDS_MITIGATION_OFF]	= "Vulnerable",
-	[MDS_MITIGATION_FULL]	= "Mitigation: Clear CPU buffers"
+	[MDS_MITIGATION_FULL]	= "Mitigation: Clear CPU buffers",
+	[MDS_MITIGATION_VMWERV]	= "Vulnerable: Clear CPU buffers attempted, no microcode",
 };
 
 static void __init mds_select_mitigation(void)
@@ -235,10 +236,9 @@ static void __init mds_select_mitigation(void)
 	}
 
 	if (mds_mitigation == MDS_MITIGATION_FULL) {
-		if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
-			static_branch_enable(&mds_user_clear);
-		else
-			mds_mitigation = MDS_MITIGATION_OFF;
+		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
+			mds_mitigation = MDS_MITIGATION_VMWERV;
+		static_branch_enable(&mds_user_clear);
 	}
 	pr_info("%s\n", mds_strings[mds_mitigation]);
 }
@@ -705,8 +705,14 @@ void arch_smt_update(void)
 		break;
 	}
 
-	if (mds_mitigation == MDS_MITIGATION_FULL)
+	switch (mds_mitigation) {
+	case MDS_MITIGATION_FULL:
+	case MDS_MITIGATION_VMWERV:
 		update_mds_branch_idle();
+		break;
+	case MDS_MITIGATION_OFF:
+		break;
+	}
 
 	mutex_unlock(&spec_ctrl_mutex);
 }