qemu: Add pre-migration hook

This hook is called during the Prepare phase on destination host and may
be used for changing domain XML.

docs/hooks.html.in

@@ -120,6 +120,16 @@
         called again, <span class="since">since 0.9.0</span>, to allow
         any additional resource cleanup:<br/>
         <pre>/etc/libvirt/hooks/qemu guest_name release end -</pre></li>
+      <li><span class="since">Since 0.9.11</span>, the qemu hook script
+        is also called at the beginning of incoming migration. It is called
+        as: <pre>/etc/libvirt/hooks/qemu guest_name migrate begin -</pre>
+        with domain XML sent to standard input of the script. In this case,
+        the script acts as a filter and is supposed to modify the domain
+        XML and print it out on its standard output. Empty output is
+        identical to copying the input XML without changing it. In case the
+        script returns failure or the output XML is not valid, incoming
+        migration will be canceled. This hook may be used, e.g., to change
+        location of disk images for incoming domains.</li>
     </ul>
 
     <h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
@@ -161,19 +171,20 @@
        source and destination hosts:</p>
     <ol>
       <li>At the beginning of the migration, the <i>qemu</i> hook script on
-          the <b>destination</b> host is executed with the "start"
-          operation.<br/><br/></li>
-      <li>If this hook script returns indicating success (error code 0), the
-          migration continues.  Any other return code indicates failure, and
-          the migration is aborted.<br/><br/></li>
-      <li>The QEMU guest is then migrated to the destination host.<br/>
-          <br/></li>
+          the <b>destination</b> host is executed with the "migrate"
+          operation.</li>
+      <li>Before QEMU process is spawned, the two operations ("prepare" and
+          "start") called for domain start are executed on
+          <b>destination</b> host.</li>
+      <li>If both of these hook script executions exit successfully (exit
+          status 0), the migration continues.  Any other exit code indicates
+          failure, and the migration is aborted.</li>
+      <li>The QEMU guest is then migrated to the destination host.</li>
       <li>Unless an error occurs during the migration process, the <i>qemu</i>
-          hook script on the <b>source</b> host is then executed with the "stopped"
-          operation, to indicate it is no longer running on this
-          host.<br/><br/>
-          Regardless of the return code from this hook script, the migration
-          is not aborted as it has already been performed.</li>
+          hook script on the <b>source</b> host is then executed with the
+          "stopped" and "release" operations to indicate it is no longer
+          running on this host. Regardless of the return codes, the
+          migration is not aborted as it has already been performed.</li>
     </ol>
     <br/>
 

src/qemu/qemu_migration.c

@@ -47,6 +47,7 @@
 #include "rpc/virnetsocket.h"
 #include "storage_file.h"
 #include "viruri.h"
+#include "hooks.h"
 
 
 #define VIR_FROM_THIS VIR_FROM_QEMU
@@ -1130,6 +1131,7 @@ qemuMigrationPrepareAny(struct qemud_driver *driver,
     qemuMigrationCookiePtr mig = NULL;
     bool tunnel = !!st;
     char *origname = NULL;
+    char *xmlout = NULL;
 
     if (virTimeMillisNow(&now) < 0)
         return -1;
@@ -1150,6 +1152,46 @@ qemuMigrationPrepareAny(struct qemud_driver *driver,
             goto cleanup;
     }
 
+    /* Let migration hook filter domain XML */
+    if (virHookPresent(VIR_HOOK_DRIVER_QEMU)) {
+        char *xml;
+        int hookret;
+
+        if (!(xml = virDomainDefFormat(def, VIR_DOMAIN_XML_SECURE)))
+            goto cleanup;
+
+        hookret = virHookCall(VIR_HOOK_DRIVER_QEMU, def->name,
+                              VIR_HOOK_QEMU_OP_MIGRATE, VIR_HOOK_SUBOP_BEGIN,
+                              NULL, xml, &xmlout);
+        VIR_FREE(xml);
+
+        if (hookret < 0) {
+            goto cleanup;
+        } else if (hookret == 0) {
+            if (!*xmlout) {
+                VIR_DEBUG("Migrate hook filter returned nothing; using the"
+                          " original XML");
+            } else {
+                virDomainDefPtr newdef;
+
+                VIR_DEBUG("Using hook-filtered domain XML: %s", xmlout);
+                newdef = virDomainDefParseString(driver->caps, xmlout,
+                                                 QEMU_EXPECTED_VIRT_TYPES,
+                                                 VIR_DOMAIN_XML_INACTIVE);
+                if (!newdef)
+                    goto cleanup;
+
+                if (!virDomainDefCheckABIStability(def, newdef)) {
+                    virDomainDefFree(newdef);
+                    goto cleanup;
+                }
+
+                virDomainDefFree(def);
+                def = newdef;
+            }
+        }
+    }
+
     if (virDomainObjIsDuplicate(&driver->domains, def, 1) < 0)
         goto cleanup;
 
@@ -1244,6 +1286,7 @@ qemuMigrationPrepareAny(struct qemud_driver *driver,
 
 cleanup:
     VIR_FREE(origname);
+    VIR_FREE(xmlout);
     virDomainDefFree(def);
     VIR_FORCE_CLOSE(dataFD[0]);
     VIR_FORCE_CLOSE(dataFD[1]);

src/util/hooks.c

@@ -73,7 +73,8 @@ VIR_ENUM_IMPL(virHookQemuOp, VIR_HOOK_QEMU_OP_LAST,
               "start",
               "stopped",
               "prepare",
-              "release")
+              "release",
+              "migrate")
 
 VIR_ENUM_IMPL(virHookLxcOp, VIR_HOOK_LXC_OP_LAST,
               "start",

src/util/hooks.h

@@ -56,6 +56,7 @@ enum virHookQemuOpType {
     VIR_HOOK_QEMU_OP_STOPPED,          /* domain has stopped */
     VIR_HOOK_QEMU_OP_PREPARE,          /* domain startup initiated */
     VIR_HOOK_QEMU_OP_RELEASE,          /* domain destruction is over */
+    VIR_HOOK_QEMU_OP_MIGRATE,          /* domain is being migrated */
 
     VIR_HOOK_QEMU_OP_LAST,
 };