[Xen-devel] [PATCH]: Expand Xen blkfront for > 16 xvd

[Chris Lalancette <clalance@xxxxxxxxxx>]

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@xxxxxxxxxx>
Acked-by: Jeremy Fitzhardinge <jeremy@xxxxxxxx>

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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