x86/S3: restore MCE (APs) init

MCE init for APs was broken when CPU feature re-checking was added. At the
same time make sure we don't bypass setup_doitm() (and whatever else may
be added to the bottom of identify_cpu()).

Fixes: bb502a8ca5 ("x86: check feature flags after resume")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 5ebd4dd141
master date: 2026-03-24 12:05:42 +0100

xen/arch/x86/cpu/common.c

@@ -613,16 +613,20 @@ void identify_cpu(struct cpuinfo_x86 *c)
 			       smp_processor_id());
 	}
 
-	if (system_state == SYS_STATE_resume)
-		return;
+	if (system_state == SYS_STATE_resume) {
+		unsigned int cpu = smp_processor_id();
 
+		/* The BSP has this done right from enter_state(). */
+		if (cpu)
+			mcheck_init(&cpu_data[cpu], false);
+	}
 	/*
 	 * On SMP, boot_cpu_data holds the common feature set between
 	 * all CPUs; so make sure that we indicate which features are
 	 * common between the CPUs.  The first time this routine gets
 	 * executed, c == &boot_cpu_data.
 	 */
-	if ( c != &boot_cpu_data ) {
+	else if (c != &boot_cpu_data) {
 		/* AND the already accumulated flags with these */
 		for ( i = 0 ; i < NCAPINTS ; i++ )
 			boot_cpu_data.x86_capability[i] &= c->x86_capability[i];