PCI/IOV: Skip INTx config reads for VFs

Per PCIe r4.0, sec 9.2.1.4, VFs can not implement INTX, and their Interrupt
Line and Interrupt Pin registers must be RO Zero.  Some devices have
thousands of VFs, so skip reading the registers as an optimization.

Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
[bhelgaas: changelog, comment]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>

drivers/pci/probe.c

@@ -1230,6 +1230,13 @@ static void pci_read_irq(struct pci_dev *dev)
 {
 	unsigned char irq;
 
+	/* VFs are not allowed to use INTx, so skip the config reads */
+	if (dev->is_virtfn) {
+		dev->pin = 0;
+		dev->irq = 0;
+		return;
+	}
+
 	pci_read_config_byte(dev, PCI_INTERRUPT_PIN, &irq);
 	dev->pin = irq;
 	if (irq)