vfio/pci: Pull BAR mapping setup from read-write path

This creates a common helper that we'll use for ioeventfd setup.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_rdwr.c

@@ -113,6 +113,30 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf,
 	return done;
 }
 
+static int vfio_pci_setup_barmap(struct vfio_pci_device *vdev, int bar)
+{
+	struct pci_dev *pdev = vdev->pdev;
+	int ret;
+	void __iomem *io;
+
+	if (vdev->barmap[bar])
+		return 0;
+
+	ret = pci_request_selected_regions(pdev, 1 << bar, "vfio");
+	if (ret)
+		return ret;
+
+	io = pci_iomap(pdev, bar, 0);
+	if (!io) {
+		pci_release_selected_regions(pdev, 1 << bar);
+		return -ENOMEM;
+	}
+
+	vdev->barmap[bar] = io;
+
+	return 0;
+}
+
 ssize_t vfio_pci_bar_rw(struct vfio_pci_device *vdev, char __user *buf,
 			size_t count, loff_t *ppos, bool iswrite)
 {
@@ -147,22 +171,13 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_device *vdev, char __user *buf,
 		if (!io)
 			return -ENOMEM;
 		x_end = end;
-	} else if (!vdev->barmap[bar]) {
-		int ret;
-
-		ret = pci_request_selected_regions(pdev, 1 << bar, "vfio");
+	} else {
+		int ret = vfio_pci_setup_barmap(vdev, bar);
 		if (ret)
 			return ret;
 
-		io = pci_iomap(pdev, bar, 0);
-		if (!io) {
-			pci_release_selected_regions(pdev, 1 << bar);
-			return -ENOMEM;
-		}
-
-		vdev->barmap[bar] = io;
-	} else
 		io = vdev->barmap[bar];
+	}
 
 	if (bar == vdev->msix_bar) {
 		x_start = vdev->msix_offset;