pc: Require IRQ remapping and EIM if there could be x2APIC CPUs

It would prevent starting guest with incorrect configs
where interrupts couldn't be delivered to CPUs with
APIC IDs > 255.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NRadim Krčmář <rkrcmar@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

hw/i386/pc.c

@@ -68,6 +68,7 @@
 #include "qapi-visit.h"
 #include "qom/cpu.h"
 #include "hw/nmi.h"
+#include "hw/i386/intel_iommu.h"
 
 /* debug PC/ISA interrupts */
 //#define DEBUG_IRQ
@@ -1273,6 +1274,19 @@ void pc_machine_done(Notifier *notifier, void *data)
                             sizeof(pcms->boot_cpus_le));
         }
     }
+
+    if (pcms->apic_id_limit > 255) {
+        IntelIOMMUState *iommu = INTEL_IOMMU_DEVICE(x86_iommu_get_default());
+
+        if (!iommu || !iommu->x86_iommu.intr_supported ||
+            iommu->intr_eim != ON_OFF_AUTO_ON) {
+            error_report("current -smp configuration requires "
+                         "Extended Interrupt Mode enabled. "
+                         "You can add an IOMMU using: "
+                         "-device intel-iommu,intremap=on,eim=on");
+            exit(EXIT_FAILURE);
+        }
+    }
 }
 
 void pc_guest_info_init(PCMachineState *pcms)