block: Ignore guest dev permissions during incoming migration

Usually guest devices don't like other writers to the same image, so
they use blk_set_perm() to prevent this from happening. In the migration
phase before the VM is actually running, though, they don't have a
problem with writes to the image. On the other hand, storage migration
needs to be able to write to the image in this phase, so the restrictive
blk_set_perm() call of qdev devices breaks it.

This patch flags all BlockBackends with a qdev device as
blk->disable_perm during incoming migration, which means that the
requested permissions are stored in the BlockBackend, but not actually
applied to its root node yet.

Once migration has finished and the VM should be resumed, the
permissions are applied. If they cannot be applied (e.g. because the NBD
server used for block migration hasn't been shut down), resuming the VM
fails.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>

1 files changed, 39 insertions, 1 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index 0b63773..18ece99 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -61,6 +61,7 @@ struct BlockBackend {
 
     uint64_t perm;
     uint64_t shared_perm;
+    bool disable_perm;
 
     bool allow_write_beyond_eof;
 
@@ -578,7 +579,7 @@ int blk_set_perm(BlockBackend *blk, uint64_t perm, uint64_t shared_perm,
 {
     int ret;
 
-    if (blk->root) {
+    if (blk->root && !blk->disable_perm) {
         ret = bdrv_child_try_set_perm(blk->root, perm, shared_perm, errp);
         if (ret < 0) {
             return ret;
@@ -597,15 +598,52 @@ void blk_get_perm(BlockBackend *blk, uint64_t *perm, uint64_t *shared_perm)
     *shared_perm = blk->shared_perm;
 }
 
+/*
+ * Notifies the user of all BlockBackends that migration has completed. qdev
+ * devices can tighten their permissions in response (specifically revoke
+ * shared write permissions that we needed for storage migration).
+ *
+ * If an error is returned, the VM cannot be allowed to be resumed.
+ */
+void blk_resume_after_migration(Error **errp)
+{
+    BlockBackend *blk;
+    Error *local_err = NULL;
+
+    for (blk = blk_all_next(NULL); blk; blk = blk_all_next(blk)) {
+        if (!blk->disable_perm) {
+            continue;
+        }
+
+        blk->disable_perm = false;
+
+        blk_set_perm(blk, blk->perm, blk->shared_perm, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            blk->disable_perm = true;
+            return;
+        }
+    }
+}
+
 static int blk_do_attach_dev(BlockBackend *blk, void *dev)
 {
     if (blk->dev) {
         return -EBUSY;
     }
+
+    /* While migration is still incoming, we don't need to apply the
+     * permissions of guest device BlockBackends. We might still have a block
+     * job or NBD server writing to the image for storage migration. */
+    if (runstate_check(RUN_STATE_INMIGRATE)) {
+        blk->disable_perm = true;
+    }
+
     blk_ref(blk);
     blk->dev = dev;
     blk->legacy_dev = false;
     blk_iostatus_reset(blk);
+
     return 0;
 }