Index: sys/x86/x86/mp_x86.c
===================================================================
--- sys/x86/x86/mp_x86.c
+++ sys/x86/x86/mp_x86.c
@@ -153,6 +153,10 @@
 	&hyperthreading_intr_allowed, 0,
 	"Allow interrupts on HTT logical CPUs");
 
+static int	apic_id_limit = -1;
+SYSCTL_INT(_machdep, OID_AUTO, apic_id_limit, CTLFLAG_RDTUN,
+	&apic_id_limit, 0, "Maximum permitted APIC ID (-1 is unlimited)");
+
 static struct topo_node topo_root;
 
 static int pkg_id_shift;
@@ -258,6 +262,10 @@
 	if ((amd_feature2 & AMDID2_CMP) == 0)
 		return;
 
+	/* XXX Lack of an AMD IOMMU driver prevents use of APIC IDs over 255. */
+	if (apic_id_limit == -1)
+		apic_id_limit = 255;
+
 	/* For families 10h and newer. */
 	pkg_id_shift = (cpu_procinfo2 & AMDID_COREID_SIZE) >>
 	    AMDID_COREID_SIZE_SHIFT;
@@ -1165,6 +1173,8 @@
 			continue;
 		if (cpu_info[apic_id].cpu_disabled)
 			continue;
+		if (apic_id_limit >= 0 && apic_id > apic_id_limit)
+			continue;
 
 		/* Don't let hyperthreads service interrupts. */
 		if (cpu_info[apic_id].cpu_hyperthread &&