blktap2: fix save/restore/migration

blktap2 devices use a regular 'phy' vbd blkback backend, causing
Blktap2Controller to trample the devices' parameters. This causes
problems with save/restore and managed domains, among other
things. This patch modifies Blktap2Controller to store both the vbd
and tap2 parameters in xenstore, and stops it from trampling the
device's config on device creation.

 * store blktap2 parameters in xenstore
 * restore blktap2 device config to original state once the underlying
 * vbd
   device is created (this fixes managed domains)
 * use blktap2 parameters rather than vbd parameters when building
 * blktap2
   device configurations
 * remove blktap2 specific code from XendConfig

Signed-off-by: Ryan O'Connor <rjo@cs.ubc.ca>

tools/python/xen/xend/XendConfig.py

@@ -1120,8 +1120,6 @@ class XendConfig(dict):
                                     if sxp.child_value(config, 'bootable', None) is None:
                                         is_bootable = dev_cfg.get('bootable', 0)
                                         config.append(['bootable', int(is_bootable)])
-                                    if dev_cfg.has_key('required_uname'):
-                                        config.append(['required_uname', dev_cfg['required_uname']])
                                     config.append(['VDI', dev_cfg.get('VDI', '')])
 
                                 sxpr.append(['device', config])
@@ -1372,13 +1370,6 @@ class XendConfig(dict):
                     dev_info['driver'] = 'paravirtualised'
 
             if dev_type == 'tap' or dev_type == 'tap2':
-                if dev_info.has_key('required_uname'):
-                    # Restore uname by required_uname because uname might
-                    # be replaced with 'phy:/dev/xen/blktap-2/tapdev*'.
-                    dev_info['uname'] = dev_info['required_uname']
-                else:
-                    # Save uname for next domain start.
-                    dev_info['required_uname'] = dev_info['uname']
                 tap_disk_type = dev_info['uname'].split(':')[1]
                 # tapdisk uname may be 'tap:<driver>' or 'tap:tapdisk:<driver>'
                 if tap_disk_type == 'tapdisk':

tools/python/xen/xend/server/BlktapController.py

@@ -137,10 +137,39 @@ class Blktap2Controller(BlktapController):
                                         deviceClass,
                                         self.vm.getDomid(), devid)
 
+    def getDeviceDetails(self, config):
+
+        (devid, back, front) = BlktapController.getDeviceDetails(self, config)
+        if self.deviceClass == 'tap2':
+        # since blktap2 uses blkback as a backend the 'params' feild contains
+        # the path to the blktap2 device (/dev/xen/blktap-2/tapdev*). As well,
+        # we need to store the params used to create the blktap2 device
+        # (tap:tapdisk:<driver>:/<image-path>)
+            tapdisk_uname = config.get('tapdisk_uname', '')
+            (_, tapdisk_params) = string.split(tapdisk_uname, ':', 1)
+            back['tapdisk-params'] = tapdisk_params 
+            
+        return (devid, back, front)
+
+    def getDeviceConfiguration(self, devid, transaction = None):
+
+        # this is a blktap2 device, so we need to overwrite the 'params' feild
+        # with the actual blktap2 parameters. (the vbd parameters are of little
+        # use to us)
+        config = BlktapController.getDeviceConfiguration(self, devid, transaction)
+        if transaction is None:
+            tapdisk_params = self.readBackend(devid, 'tapdisk-params')
+        else:
+            tapdisk_params = self.readBackendTxn(transaction, devid, 'tapdisk-params')
+        if tapdisk_params:
+            config['uname'] = 'tap:' + tapdisk_params
+
+        return config
+
 
     def createDevice(self, config):
 
-        uname = config.get('required_uname', '')
+        uname = config.get('uname', '')
         try:
             (typ, subtyp, params, file) = string.split(uname, ':', 3)
         except:
@@ -180,11 +209,15 @@ class Blktap2Controller(BlktapController):
         stdout.close();
         stderr.close();
 
-        #modify the configuration to attach as a vbd, now that the
-        #device is configured.  Then continue to create the device
+        # modify the configutration to create a blkback for the underlying
+        # blktap2 device. Note: we need to preserve the original tapdisk uname
+        # (it is used during save/restore and for managed domains).
+        config.update({'tapdisk_uname' : uname})
         config.update({'uname' : 'phy:' + device.rstrip()})
 
         devid = BlkifController.createDevice(self, config)
+        config.update({'uname' : uname})
+        config.pop('tapdisk_uname')
         return devid
 
     # The new blocktap implementation requires a sysfs signal to close