x86/mce: WARN once if IA32_FEAT_CTL MSR is left unlocked

WARN if the IA32_FEAT_CTL MSR is somehow left unlocked now that CPU
initialization unconditionally locks the MSR.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20191221044513.21680-6-sean.j.christopherson@intel.com

arch/x86/kernel/cpu/mce/intel.c

@@ -116,14 +116,15 @@ static bool lmce_supported(void)
 	/*
 	 * BIOS should indicate support for LMCE by setting bit 20 in
 	 * IA32_FEAT_CTL without which touching MCG_EXT_CTL will generate a #GP
-	 * fault.
+	 * fault.  The MSR must also be locked for LMCE_ENABLED to take effect.
+	 * WARN if the MSR isn't locked as init_ia32_feat_ctl() unconditionally
+	 * locks the MSR in the event that it wasn't already locked by BIOS.
 	 */
 	rdmsrl(MSR_IA32_FEAT_CTL, tmp);
-	if ((tmp & (FEAT_CTL_LOCKED | FEAT_CTL_LMCE_ENABLED)) ==
-		   (FEAT_CTL_LOCKED | FEAT_CTL_LMCE_ENABLED))
-		return true;
+	if (WARN_ON_ONCE(!(tmp & FEAT_CTL_LOCKED)))
+		return false;
 
-	return false;
+	return tmp & FEAT_CTL_LMCE_ENABLED;
 }
 
 bool mce_intel_cmci_poll(void)