xen/pt: allow passthrough of devices with bogus interrupt pin

For some pci device, even its PCI_INTERRUPT_PIN is not 0, it actually
doesn't support INTx mode, so its machine irq read from host sysfs is 0.
In that case, report PCI_INTERRUPT_PIN as 0 to guest and let passthrough
continue.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

hw/xen/xen_pt.c

@@ -847,6 +847,12 @@ static void xen_pt_realize(PCIDevice *d, Error **errp)
     }
 
     machine_irq = s->real_device.irq;
+    if (machine_irq == 0) {
+        XEN_PT_LOG(d, "machine irq is 0\n");
+        cmd |= PCI_COMMAND_INTX_DISABLE;
+        goto out;
+    }
+
     rc = xc_physdev_map_pirq(xen_xc, xen_domid, machine_irq, &pirq);
     if (rc < 0) {
         error_setg_errno(errp, errno, "Mapping machine irq %u to"

hw/xen/xen_pt_config_init.c

@@ -300,7 +300,9 @@ static int xen_pt_irqpin_reg_init(XenPCIPassthroughState *s,
                                   XenPTRegInfo *reg, uint32_t real_offset,
                                   uint32_t *data)
 {
-    *data = xen_pt_pci_read_intx(s);
+    if (s->real_device.irq) {
+        *data = xen_pt_pci_read_intx(s);
+    }
     return 0;
 }