ivshmem: Leave INTx alone when using MSI-X

The ivshmem device can either use MSI-X or legacy INTx for interrupts.

With MSI-X enabled, peer interrupt events trigger an MSI as they
should.  But software can still raise INTx via interrupt status and
mask register in BAR 0.  This is explicitly prohibited by PCI Local
Bus Specification Revision 3.0, section 6.8.3.3:

    While enabled for MSI or MSI-X operation, a function is prohibited
    from using its INTx# pin (if implemented) to request service (MSI,
    MSI-X, and INTx# are mutually exclusive).

Fix the device model to leave INTx alone when using MSI-X.

Document that we claim to use INTx in config space even when we don't.
Unlike other devices, ivshmem does *not* use INTx when configured for
MSI-X and MSI-X isn't enabled by software.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1458066895-20632-21-git-send-email-armbru@redhat.com>

hw/misc/ivshmem.c

@@ -126,6 +126,11 @@ static void ivshmem_update_irq(IVShmemState *s)
     PCIDevice *d = PCI_DEVICE(s);
     uint32_t isr = s->intrstatus & s->intrmask;
 
+    /* No INTx with msi=on, whether the guest enabled MSI-X or not */
+    if (ivshmem_has_feature(s, IVSHMEM_MSI)) {
+        return;
+    }
+
     /* don't print ISR resets */
     if (isr) {
         IVSHMEM_DPRINTF("Set IRQ to %d (%04x %04x)\n",
@@ -873,6 +878,10 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error **errp)
     pci_conf = dev->config;
     pci_conf[PCI_COMMAND] = PCI_COMMAND_IO | PCI_COMMAND_MEMORY;
 
+    /*
+     * Note: we don't use INTx with IVSHMEM_MSI at all, so this is a
+     * bald-faced lie then.  But it's a backwards compatible lie.
+     */
     pci_config_set_interrupt_pin(pci_conf, 1);
 
     memory_region_init_io(&s->ivshmem_mmio, OBJECT(s), &ivshmem_mmio_ops, s,