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Subject[PATCH]: Expand Xen blkfront for > 16 xvd
Until recently, the maximum number of xvd block devices you could attach to
a Xen domU was 16. This limitation turned out to be problematic for some users,
so it was expanded to handle a much larger number of disks. However, this
requires a couple of changes in the way that blkfront scans for disks. This
functionality is already present in the Xen linux-2.6.18-xen.hg tree; the
attached patch adds this functionality to the mainline xen-blkfront
implementation. I successfully tested it on a 2.6.25 tree. I build tested it
on 2.6.27-rc3, but couldn't get that tree to boot due to some other bug.

Signed-off-by: Chris Lalancette <clalance@redhat.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 3ca643c..bff602c 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -105,15 +105,17 @@ static DEFINE_SPINLOCK(blkif_io_lock);
#define GRANT_INVALID_REF 0

#define PARTS_PER_DISK 16
+#define PARTS_PER_EXT_DISK 256

#define BLKIF_MAJOR(dev) ((dev)>>8)
#define BLKIF_MINOR(dev) ((dev) & 0xff)

-#define DEV_NAME "xvd" /* name in /dev */
+#define EXT_SHIFT 28
+#define EXTENDED (1<<EXT_SHIFT)
+#define VDEV_IS_EXTENDED(dev) ((dev)&(EXTENDED))
+#define BLKIF_MINOR_EXT(dev) ((dev)&(~EXTENDED))

-/* Information about our VBDs. */
-#define MAX_VBDS 64
-static LIST_HEAD(vbds_list);
+#define DEV_NAME "xvd" /* name in /dev */

static int get_id_from_freelist(struct blkfront_info *info)
{
@@ -386,31 +388,60 @@ static int xlvbd_barrier(struct blkfront_info *info)
}


-static int xlvbd_alloc_gendisk(int minor, blkif_sector_t capacity,
- int vdevice, u16 vdisk_info, u16 sector_size,
- struct blkfront_info *info)
+static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
+ struct blkfront_info *info,
+ u16 vdisk_info, u16 sector_size)
{
struct gendisk *gd;
int nr_minors = 1;
int err = -ENODEV;
+ unsigned int offset;
+ int minor;
+ int nr_parts;

BUG_ON(info->gd != NULL);
BUG_ON(info->rq != NULL);

- if ((minor % PARTS_PER_DISK) == 0)
- nr_minors = PARTS_PER_DISK;
+ if ((info->vdevice>>EXT_SHIFT) > 1) {
+ /* this is above the extended range; something is wrong */
+ printk(KERN_WARNING "blkfront: vdevice 0x%x is above the extended range; ignoring\n", info->vdevice);
+ return -ENODEV;
+ }
+
+ if (!VDEV_IS_EXTENDED(info->vdevice)) {
+ minor = BLKIF_MINOR(info->vdevice);
+ nr_parts = PARTS_PER_DISK;
+ } else {
+ minor = BLKIF_MINOR_EXT(info->vdevice);
+ nr_parts = PARTS_PER_EXT_DISK;
+ }
+
+ if ((minor % nr_parts) == 0)
+ nr_minors = nr_parts;

gd = alloc_disk(nr_minors);
if (gd == NULL)
goto out;

- if (nr_minors > 1)
- sprintf(gd->disk_name, "%s%c", DEV_NAME,
- 'a' + minor / PARTS_PER_DISK);
- else
- sprintf(gd->disk_name, "%s%c%d", DEV_NAME,
- 'a' + minor / PARTS_PER_DISK,
- minor % PARTS_PER_DISK);
+ offset = minor / nr_parts;
+
+ if (nr_minors > 1) {
+ if (offset < 26)
+ sprintf(gd->disk_name, "%s%c", DEV_NAME, 'a' + offset);
+ else
+ sprintf(gd->disk_name, "%s%c%c", DEV_NAME,
+ 'a' + ((offset / 26)-1), 'a' + (offset % 26));
+ } else {
+ if (offset < 26)
+ sprintf(gd->disk_name, "%s%c%d", DEV_NAME,
+ 'a' + offset,
+ minor & (nr_parts - 1));
+ else
+ sprintf(gd->disk_name, "%s%c%c%d", DEV_NAME,
+ 'a' + ((offset / 26) - 1),
+ 'a' + (offset % 26),
+ minor & (nr_parts - 1));
+ }

gd->major = XENVBD_MAJOR;
gd->first_minor = minor;
@@ -699,8 +730,13 @@ static int blkfront_probe(struct xenbus_device *dev,
err = xenbus_scanf(XBT_NIL, dev->nodename,
"virtual-device", "%i", &vdevice);
if (err != 1) {
- xenbus_dev_fatal(dev, err, "reading virtual-device");
- return err;
+ /* go looking in the extended area instead */
+ err = xenbus_scanf(XBT_NIL, dev->nodename, "virtual-device-ext",
+ "%i", &vdevice);
+ if (err != 1) {
+ xenbus_dev_fatal(dev, err, "reading virtual-device");
+ return err;
+ }
}

info = kzalloc(sizeof(*info), GFP_KERNEL);
@@ -861,9 +897,7 @@ static void blkfront_connect(struct blkfront_info *info)
if (err)
info->feature_barrier = 0;

- err = xlvbd_alloc_gendisk(BLKIF_MINOR(info->vdevice),
- sectors, info->vdevice,
- binfo, sector_size, info);
+ err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
if (err) {
xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
info->xbdev->otherend);
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