virtio-pci: disable msi at startup

commit b03214d5 upstream.

virtio-pci resets the device at startup by writing to the status
register, but this does not clear the pci config space,
specifically msi enable status which affects register
layout.

This breaks things like kdump when they try to use e.g. virtio-blk.

Fix by forcing msi off at startup. Since pci.c already has
a routine to do this, we export and use it instead of duplicating code.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: linux-pci@vger.kernel.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/pci/pci.c

@@ -2046,6 +2046,7 @@ void pci_msi_off(struct pci_dev *dev)
 		pci_write_config_word(dev, pos + PCI_MSIX_FLAGS, control);
 	}
 }
+EXPORT_SYMBOL_GPL(pci_msi_off);
 
 #ifndef HAVE_ARCH_PCI_SET_DMA_MASK
 /*

drivers/virtio/virtio_pci.c

@@ -635,6 +635,9 @@ static int __devinit virtio_pci_probe(struct pci_dev *pci_dev,
 	INIT_LIST_HEAD(&vp_dev->virtqueues);
 	spin_lock_init(&vp_dev->lock);
 
+	/* Disable MSI/MSIX to bring device to a known good state. */
+	pci_msi_off(pci_dev);
+
 	/* enable the device */
 	err = pci_enable_device(pci_dev);
 	if (err)