[PATCH v2 5/6] 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               | 64 ++++++++++++++++++++++++++--------
 include/block/snapshot.h       |  4 ++-
 migration/savevm.c             |  4 +--
 4 files changed, 56 insertions(+), 18 deletions(-)

diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c
index db76c43cc2..81d1b52262 100644
--- a/block/monitor/block-hmp-cmds.c
+++ b/block/monitor/block-hmp-cmds.c
@@ -900,7 +900,7 @@ void hmp_info_snapshots(Monitor *mon, const QDict *qdict)
     SnapshotEntry *snapshot_entry;
     Error *err = NULL;
 
-    bs = bdrv_all_find_vmstate_bs(NULL, &err);
+    bs = bdrv_all_find_vmstate_bs(NULL, NULL, &err);
     if (!bs) {
         error_report_err(err);
         return;
diff --git a/block/snapshot.c b/block/snapshot.c
index f2600a8c7f..b1ad70e278 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -551,27 +551,63 @@ int bdrv_all_create_snapshot(QEMUSnapshotInfo *sn,
     return 0;
 }
 
-BlockDriverState *bdrv_all_find_vmstate_bs(strList *devices, Error **errp)
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           strList *devices,
+                                           Error **errp)
 {
     BlockDriverState *bs;
     BdrvNextIterator it;
 
-    for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
-        AioContext *ctx = bdrv_get_aio_context(bs);
-        bool found;
+    if (vmstate_bs) {
+        bool usable = false;
+        for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
+            AioContext *ctx = bdrv_get_aio_context(bs);
+            bool match;
 
-        aio_context_acquire(ctx);
-        found = bdrv_all_snapshots_includes_bs(bs, devices) &&
-            bdrv_can_snapshot(bs);
-        aio_context_release(ctx);
+            aio_context_acquire(ctx);
+            if (g_str_equal(vmstate_bs, bdrv_get_node_name(bs))) {
+                match = true;
+                usable = bdrv_can_snapshot(bs);
+            }
+            aio_context_release(ctx);
+            if (match) {
+                bdrv_next_cleanup(&it);
+                break;
+            }
+        }
+        if (!bs) {
+            error_setg(errp,
+                       "block device '%s' does not exist",
+                       vmstate_bs);
+            return NULL;
+        }
 
-        if (found) {
-            bdrv_next_cleanup(&it);
-            break;
+        if (!usable) {
+            error_setg(errp,
+                       "block device '%s' does not support snapshots",
+                       vmstate_bs);
+            return NULL;
+        }
+    } else {
+        for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
+            AioContext *ctx = bdrv_get_aio_context(bs);
+            bool found;
+
+            aio_context_acquire(ctx);
+            found = bdrv_all_snapshots_includes_bs(bs, devices) &&
+                bdrv_can_snapshot(bs);
+            aio_context_release(ctx);
+
+            if (found) {
+                bdrv_next_cleanup(&it);
+                break;
+            }
+        }
+
+        if (!bs) {
+            error_setg(errp, "No block device supports snapshots");
+            return NULL;
         }
-    }
-    if (!bs) {
-        error_setg(errp, "No block device supports snapshots");
     }
     return bs;
 }
diff --git a/include/block/snapshot.h b/include/block/snapshot.h
index 1c5b0705a9..05550e5da1 100644
--- a/include/block/snapshot.h
+++ b/include/block/snapshot.h
@@ -86,6 +86,8 @@ int bdrv_all_create_snapshot(QEMUSnapshotInfo *sn,
                              strList *devices,
                              Error **errp);
 
-BlockDriverState *bdrv_all_find_vmstate_bs(strList *devices, Error **errp);
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           strList *devices,
+                                           Error **errp);
 
 #endif
diff --git a/migration/savevm.c b/migration/savevm.c
index cdc1f2f2d8..1707fa30db 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2664,7 +2664,7 @@ int save_snapshot(const char *name, Error **errp)
         }
     }
 
-    bs = bdrv_all_find_vmstate_bs(NULL, errp);
+    bs = bdrv_all_find_vmstate_bs(NULL, NULL, errp);
     if (bs == NULL) {
         return ret;
     }
@@ -2854,7 +2854,7 @@ int load_snapshot(const char *name, Error **errp)
         return -1;
     }
 
-    bs_vm_state = bdrv_all_find_vmstate_bs(NULL, errp);
+    bs_vm_state = bdrv_all_find_vmstate_bs(NULL, NULL, errp);
     if (!bs_vm_state) {
         return -1;
     }
-- 
2.26.2