CA-11936: Automatically plug block-based RAW VDIs via blkback

This patch causes block-based (LV) RAW VDIs to be automatically plugged by
blkback. This is achieved as follows:

During VDI.attach, we detect that the SR handles LVM and that the VDI driver
is 'aio'. In that case, we write 'backend-kind: vbd' to xenstore_data, which
causes the toolstack to use the /local/domain/0/backend/vbd/ path instead of
.../vbd3/. This is the preferred method following CP-11511. By writing to
.../vbd/, blkback wakes up instead of tapback.

During VDI.activate, we create the /dev/sm/backend/ node using the maj/min
numbers of the actual VDI (LV) instead of those of the blktap2 device. This
causes the udev hotplug scripts to write the /physical-device/ entry with
information of the LV, creating the blkfront-blkback-vdi datapath without
blktap2 or tapdisk3 in the middle.

Note that a tapdisk3 and a blktap2 will still exist in dom0, providing all
necessary backend infrastructure resources for other operations.

Signed-off-by: Felipe Franciosi <[email protected]>

drivers/blktap2.py

@@ -1184,6 +1184,12 @@ class SymLink(Link):
         def readlink(self):
             return os.readlink(self.path())
 
+        def realpath(self):
+            return os.path.realpath(self.path())
+
+        def is_block(self):
+            return S_ISBLK(os.stat(self.realpath()).st_mode)
+
         def symlink(self):
             return self.path
 
@@ -1487,6 +1493,13 @@ def attach(self, sr_uuid, vdi_uuid, writable, activate = False):
                     {"rdonly": not writable})
             self.BackendLink.from_uuid(sr_uuid, vdi_uuid).mklink(dev_path)
 
+        # Decide whether to use tapdisk3 or blkback
+        if (self.target.get_vdi_type() == "aio") and \
+           (self.target.vdi.sr.handles("lvm")):
+            self.xenstore_data['backend-kind'] = 'vbd'
+        else:
+            self.xenstore_data['backend-kind'] = 'vbd3'
+
         # Return backend/ link
         back_path = self.BackendLink.from_uuid(sr_uuid, vdi_uuid).path()
         struct = { 'params': back_path,
@@ -1571,8 +1584,14 @@ def _activate_locked(self, sr_uuid, vdi_uuid, options):
                         break
             raise
 
+        # Link backend directly to RAW LV VDIs
+        phy_dev = self.PhyLink.from_uuid(sr_uuid, vdi_uuid)
+        if phy_dev.is_block() and self.target.get_vdi_type() == "aio":
+            dev_path = phy_dev.realpath()
+
         # Link result to backend/
         self.BackendLink.from_uuid(sr_uuid, vdi_uuid).mklink(dev_path)
+
         return True
 
     def _activate(self, sr_uuid, vdi_uuid, options):