virtio-pci: disable msi at startup

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>
Cc: stable@kernel.org

drivers/pci/pci.c

@@ -2292,6 +2292,7 @@ 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_MAX_SEGMENT_SIZE
 int pci_set_dma_max_seg_size(struct pci_dev *dev, unsigned int size)

drivers/virtio/virtio_pci.c

@@ -636,6 +636,9 @@ 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)