qemu: Introduce shared_filesystems configuration option

As explained in the comment, this can help in scenarios where
a shared filesystem can't be detected as such by libvirt, by
giving the admin the opportunity to provide this information
manually.

https://issues.redhat.com/browse/RHEL-35752

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/libvirtd_qemu.aug

@@ -143,6 +143,8 @@ module Libvirtd_qemu =
 
    let storage_entry = bool_entry "storage_use_nbdkit"
 
+   let filesystem_entry = str_array_entry "shared_filesystems"
+
    (* Entries that used to exist in the config which are now
     * deleted. We keep on parsing them so we don't break
     * ability to parse old configs after upgrade
@@ -173,6 +175,7 @@ module Libvirtd_qemu =
              | swtpm_entry
              | capability_filters_entry
              | storage_entry
+             | filesystem_entry
              | obsolete_entry
 
    let comment = [ label "#comment" . del /#[ \t]*/ "# " .  store /([^ \t\n][^\n]*)?/ . del /\n/ "\n" ]

src/qemu/qemu.conf.in

@@ -985,3 +985,29 @@
 # note that the default might change in future releases.
 #
 #storage_use_nbdkit = @USE_NBDKIT_DEFAULT@
+
+# libvirt will normally prevent migration if the storage backing the VM is not
+# on a shared filesystems. Sometimes, however, the storage *is* shared despite
+# not being detected as such: for example, this is the case when one of the
+# hosts involved in the migration is exporting its local storage to the other
+# one via NFS.
+#
+# Any directory listed here will be assumed to live on a shared filesystem,
+# making migration possible in scenarios such as the one described above. It's
+# the system's administrator responsibility to ensure that other hosts can
+# access this directory.
+#
+# This option is not symmetrical and should only be used on hosts where the
+# storage is being exported from. It must not be used on hosts accessing the
+# storage via a remote protocol.
+#
+# NOTE: this option is intended to help in very specific scenarios that are
+# rarely encountered. If you find yourself reaching for this option, consider
+# reworking your environment so that it follows a more common architecture
+# rather than using it.
+#
+#shared_filesystems = [
+#  "/path/to/images",
+#  "/path/to/nvram",
+#  "/path/to/swtpm"
+#]

src/qemu/qemu_conf.c

@@ -386,6 +386,8 @@ static void virQEMUDriverConfigDispose(void *obj)
 
     g_strfreev(cfg->capabilityfilters);
 
+    g_strfreev(cfg->sharedFilesystems);
+
     g_free(cfg->deprecationBehavior);
 }
 
@@ -1096,6 +1098,32 @@ virQEMUDriverConfigLoadStorageEntry(virQEMUDriverConfig *cfg,
 }
 
 
+static int
+virQEMUDriverConfigLoadFilesystemEntry(virQEMUDriverConfig *cfg,
+                                       virConf *conf)
+{
+    char **iter;
+
+    if (virConfGetValueStringList(conf, "shared_filesystems", false,
+                                  &cfg->sharedFilesystems) < 0)
+        return -1;
+
+    if (!cfg->sharedFilesystems)
+        return 0;
+
+    /* The paths provided by the user might contain trailing slashes
+     * and other fun diversions, which would break the naive string
+     * comparisons that we're later going to use them for */
+    for (iter = cfg->sharedFilesystems; *iter; iter++) {
+        char *canon = virFileCanonicalizePath(*iter);
+        g_free(*iter);
+        *iter = canon;
+    }
+
+    return 0;
+}
+
+
 int virQEMUDriverConfigLoadFile(virQEMUDriverConfig *cfg,
                                 const char *filename,
                                 bool privileged)
@@ -1170,6 +1198,9 @@ int virQEMUDriverConfigLoadFile(virQEMUDriverConfig *cfg,
     if (virQEMUDriverConfigLoadStorageEntry(cfg, conf) < 0)
         return -1;
 
+    if (virQEMUDriverConfigLoadFilesystemEntry(cfg, conf) < 0)
+        return -1;
+
     return 0;
 }
 

src/qemu/qemu_conf.h

@@ -233,6 +233,8 @@ struct _virQEMUDriverConfig {
     bool storageUseNbdkit;
 
     virQEMUSchedCore schedCore;
+
+    char **sharedFilesystems;
 };
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(virQEMUDriverConfig, virObjectUnref);

src/qemu/test_libvirtd_qemu.aug.in

@@ -119,3 +119,8 @@ module Test_libvirtd_qemu =
 { "deprecation_behavior" = "none" }
 { "sched_core" = "none" }
 { "storage_use_nbdkit" = "@USE_NBDKIT_DEFAULT@" }
+{ "shared_filesystems"
+    { "1" = "/path/to/images" }
+    { "2" = "/path/to/nvram" }
+    { "3" = "/path/to/swtpm" }
+}