xen: handled remapped IRQs when enabling a pcifront PCI device.

This happens to not be an issue currently because we take pains to try
to ensure that the GSI-IRQ mapping is 1-1 in a PV guest and that
regular event channels do not clash. However a subsequent patch is
going to break this 1-1 mapping.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>

arch/x86/pci/xen.c

@@ -226,21 +226,27 @@ static int xen_pcifront_enable_irq(struct pci_dev *dev)
 {
 	int rc;
 	int share = 1;
+	u8 gsi;
 
-	dev_info(&dev->dev, "Xen PCI enabling IRQ: %d\n", dev->irq);
-
-	if (dev->irq < 0)
-		return -EINVAL;
+	rc = pci_read_config_byte(dev, PCI_INTERRUPT_LINE, &gsi);
+	if (rc < 0) {
+		dev_warn(&dev->dev, "Xen PCI: failed to read interrupt line: %d\n",
+			 rc);
+		return rc;
+	}
 
-	if (dev->irq < NR_IRQS_LEGACY)
+	if (gsi < NR_IRQS_LEGACY)
 		share = 0;
 
-	rc = xen_allocate_pirq(dev->irq, share, "pcifront");
+	rc = xen_allocate_pirq(gsi, share, "pcifront");
 	if (rc < 0) {
-		dev_warn(&dev->dev, "Xen PCI IRQ: %d, failed to register:%d\n",
-			 dev->irq, rc);
+		dev_warn(&dev->dev, "Xen PCI: failed to register GSI%d: %d\n",
+			 gsi, rc);
 		return rc;
 	}
+
+	dev->irq = rc;
+	dev_info(&dev->dev, "Xen PCI mapped GSI%d to IRQ%d\n", gsi, dev->irq);
 	return 0;
 }