[PATCH 4/6] block: allow specifying name of block device for vmstate storage

["Daniel P. Berrangé" <berrange@redhat.com>]

Currently the vmstate will be stored in the first block device that
supports snapshots. Historically this would have usually been the
root device, but with UEFI it might be the variable store. There
needs to be a way to override the choice of block device to store
the state in.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 block/monitor/block-hmp-cmds.c |  2 +-
 block/snapshot.c               | 33 +++++++++++++++++++++++++++++----
 include/block/snapshot.h       |  4 +++-
 migration/savevm.c             |  6 ++----
 4 files changed, 35 insertions(+), 10 deletions(-)

diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c
index 0ee6e7a4be..a698fac027 100644
--- a/block/monitor/block-hmp-cmds.c
+++ b/block/monitor/block-hmp-cmds.c
@@ -899,7 +899,7 @@ void hmp_info_snapshots(Monitor *mon, const QDict *qdict)
     ImageEntry *image_entry, *next_ie;
     SnapshotEntry *snapshot_entry;
 
-    bs = bdrv_all_find_vmstate_bs(NULL);
+    bs = bdrv_all_find_vmstate_bs(NULL, NULL, NULL);
     if (!bs) {
         monitor_printf(mon, "No available block device supports snapshots\n");
         return;
diff --git a/block/snapshot.c b/block/snapshot.c
index c8950b0b86..4774a1890d 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -552,7 +552,9 @@ fail:
     return err;
 }
 
-BlockDriverState *bdrv_all_find_vmstate_bs(strList *exclude_bs)
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           strList *exclude_bs,
+                                           Error **errp)
 {
     BlockDriverState *bs;
     BdrvNextIterator it;
@@ -560,16 +562,39 @@ BlockDriverState *bdrv_all_find_vmstate_bs(strList *exclude_bs)
     for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
         AioContext *ctx = bdrv_get_aio_context(bs);
         bool found;
+        Error *err = NULL;
 
         aio_context_acquire(ctx);
-        found = bdrv_all_snapshots_includes_bs(bs, exclude_bs) &&
-            bdrv_can_snapshot(bs);
+        if (vmstate_bs == NULL) {
+            found = bdrv_all_snapshots_includes_bs(bs, exclude_bs) &&
+                bdrv_can_snapshot(bs);
+        } else {
+            if (g_str_equal(vmstate_bs, bdrv_get_node_name(bs))) {
+                found = bdrv_all_snapshots_includes_bs(bs, exclude_bs) &&
+                    bdrv_can_snapshot(bs);
+                if (!found) {
+                    error_setg(&err,
+                               "block device '%s' cannot accept snapshots",
+                               vmstate_bs);
+                }
+            } else {
+                found = false;
+            }
+        }
         aio_context_release(ctx);
 
-        if (found) {
+        if (found || err) {
             bdrv_next_cleanup(&it);
+            if (err) {
+                error_propagate(errp, err);
+                return NULL;
+            }
             break;
         }
     }
+
+    if (!bs) {
+        error_setg(errp, "No block device can accept snapshots");
+    }
     return bs;
 }
diff --git a/include/block/snapshot.h b/include/block/snapshot.h
index f20986ca37..ff627df5cb 100644
--- a/include/block/snapshot.h
+++ b/include/block/snapshot.h
@@ -90,6 +90,8 @@ int bdrv_all_create_snapshot(QEMUSnapshotInfo *sn,
                              strList *exclude_bs,
                              BlockDriverState **first_bad_bs);
 
-BlockDriverState *bdrv_all_find_vmstate_bs(strList *exclude_bs);
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           strList *exclude_bs,
+                                           Error **errp);
 
 #endif
diff --git a/migration/savevm.c b/migration/savevm.c
index 4251aa0dde..b11c6a882d 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2662,9 +2662,8 @@ int save_snapshot(const char *name, Error **errp)
         }
     }
 
-    bs = bdrv_all_find_vmstate_bs(NULL);
+    bs = bdrv_all_find_vmstate_bs(NULL, NULL, errp);
     if (bs == NULL) {
-        error_setg(errp, "No block device can accept snapshots");
         return ret;
     }
     aio_context = bdrv_get_aio_context(bs);
@@ -2857,9 +2856,8 @@ int load_snapshot(const char *name, Error **errp)
         return ret;
     }
 
-    bs_vm_state = bdrv_all_find_vmstate_bs(NULL);
+    bs_vm_state = bdrv_all_find_vmstate_bs(NULL, NULL, errp);
     if (!bs_vm_state) {
-        error_setg(errp, "No block device supports snapshots");
         return -ENOTSUP;
     }
     aio_context = bdrv_get_aio_context(bs_vm_state);
-- 
2.26.2