hw/9pfs: use migration blockers to prevent live migration when virtfs export path is mounted

Now when you try to migrate with VirtFS export path mounted, you get a proper QMP error:

(qemu) migrate tcp:localhost:4444
Migration is disabled when VirtFS export path '/tmp/' is mounted in the guest using mount_tag 'v_tmp'
(qemu)

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

hw/9pfs/virtio-9p-device.c

@@ -33,13 +33,15 @@ static V9fsState *to_virtio_9p(VirtIODevice *vdev)
 
 static void virtio_9p_get_config(VirtIODevice *vdev, uint8_t *config)
 {
+    int len;
     struct virtio_9p_config *cfg;
     V9fsState *s = to_virtio_9p(vdev);
 
-    cfg = g_malloc0(sizeof(struct virtio_9p_config) +
-                        s->tag_len);
-    stw_raw(&cfg->tag_len, s->tag_len);
-    memcpy(cfg->tag, s->tag, s->tag_len);
+    len = strlen(s->tag);
+    cfg = g_malloc0(sizeof(struct virtio_9p_config) + len);
+    stw_raw(&cfg->tag_len, len);
+    /* We don't copy the terminating null to config space */
+    memcpy(cfg->tag, s->tag, len);
     memcpy(config, cfg, s->config_size);
     g_free(cfg);
 }
@@ -96,20 +98,18 @@ VirtIODevice *virtio_9p_init(DeviceState *dev, V9fsConf *conf)
     }
 
     len = strlen(conf->tag);
-    if (len > MAX_TAG_LEN) {
+    if (len > MAX_TAG_LEN - 1) {
         fprintf(stderr, "mount tag '%s' (%d bytes) is longer than "
-                "maximum (%d bytes)", conf->tag, len, MAX_TAG_LEN);
+                "maximum (%d bytes)", conf->tag, len, MAX_TAG_LEN - 1);
         exit(1);
     }
-    /* s->tag is non-NULL terminated string */
-    s->tag = g_malloc(len);
-    memcpy(s->tag, conf->tag, len);
-    s->tag_len = len;
+
+    s->tag = strdup(conf->tag);
     s->ctx.uid = -1;
 
     s->ops = fse->ops;
     s->vdev.get_features = virtio_9p_get_features;
-    s->config_size = sizeof(struct virtio_9p_config) + s->tag_len;
+    s->config_size = sizeof(struct virtio_9p_config) + len;
     s->vdev.get_config = virtio_9p_get_config;
     s->fid_list = NULL;
     qemu_co_rwlock_init(&s->rename_lock);

hw/9pfs/virtio-9p.c

@@ -23,6 +23,7 @@
 #include "virtio-9p-xattr.h"
 #include "virtio-9p-coth.h"
 #include "trace.h"
+#include "migration.h"
 
 int open_fd_hw;
 int total_open_fd;
@@ -373,6 +374,19 @@ static void put_fid(V9fsPDU *pdu, V9fsFidState *fidp)
      * Don't free the fid if it is in reclaim list
      */
     if (!fidp->ref && fidp->clunked) {
+        if (fidp->fid == pdu->s->root_fid) {
+            /*
+             * if the clunked fid is root fid then we
+             * have unmounted the fs on the client side.
+             * delete the migration blocker. Ideally, this
+             * should be hooked to transport close notification
+             */
+            if (pdu->s->migration_blocker) {
+                migrate_del_blocker(pdu->s->migration_blocker);
+                error_free(pdu->s->migration_blocker);
+                pdu->s->migration_blocker = NULL;
+            }
+        }
         free_fid(pdu, fidp);
     }
 }
@@ -1235,6 +1249,11 @@ static void v9fs_attach(void *opaque)
     err = offset;
     trace_v9fs_attach_return(pdu->tag, pdu->id,
                              qid.type, qid.version, qid.path);
+    s->root_fid = fid;
+    /* disable migration */
+    error_set(&s->migration_blocker, QERR_VIRTFS_FEATURE_BLOCKS_MIGRATION,
+              s->ctx.fs_root, s->tag);
+    migrate_add_blocker(s->migration_blocker);
 out:
     put_fid(pdu, fidp);
 out_nofid:

hw/9pfs/virtio-9p.h

@@ -246,8 +246,7 @@ typedef struct V9fsState
     V9fsFidState *fid_list;
     FileOperations *ops;
     FsContext ctx;
-    uint16_t tag_len;
-    uint8_t *tag;
+    char *tag;
     size_t config_size;
     enum p9_proto_version proto_version;
     int32_t msize;
@@ -256,6 +255,8 @@ typedef struct V9fsState
      * on rename.
      */
     CoRwlock rename_lock;
+    int32_t root_fid;
+    Error *migration_blocker;
 } V9fsState;
 
 typedef struct V9fsStatState {

qerror.c

@@ -234,6 +234,11 @@ static const QErrorStringTable qerror_table[] = {
         .desc      = "'%(device)' uses a %(format) feature which is not "
                      "supported by this qemu version: %(feature)",
     },
+    {
+        .error_fmt = QERR_VIRTFS_FEATURE_BLOCKS_MIGRATION,
+        .desc      = "Migration is disabled when VirtFS export path '%(path)' "
+                     "is mounted in the guest using mount_tag '%(tag)'",
+    },
     {
         .error_fmt = QERR_VNC_SERVER_FAILED,
         .desc      = "Could not start VNC server on %(target)",

qerror.h

@@ -192,6 +192,9 @@ QError *qobject_to_qerror(const QObject *obj);
 #define QERR_UNKNOWN_BLOCK_FORMAT_FEATURE \
     "{ 'class': 'UnknownBlockFormatFeature', 'data': { 'device': %s, 'format': %s, 'feature': %s } }"
 
+#define QERR_VIRTFS_FEATURE_BLOCKS_MIGRATION \
+    "{ 'class': 'VirtFSFeatureBlocksMigration', 'data': { 'path': %s, 'tag': %s } }"
+
 #define QERR_VNC_SERVER_FAILED \
     "{ 'class': 'VNCServerFailed', 'data': { 'target': %s } }"