block: allow specifying name of block device for vmstate storage

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.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210204124834.774401-6-berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

block/monitor/block-hmp-cmds.c

@@ -902,7 +902,7 @@ void hmp_info_snapshots(Monitor *mon, const QDict *qdict)
     SnapshotEntry *snapshot_entry;
     Error *err = NULL;
 
-    bs = bdrv_all_find_vmstate_bs(false, NULL, &err);
+    bs = bdrv_all_find_vmstate_bs(NULL, false, NULL, &err);
     if (!bs) {
         error_report_err(err);
         return;

block/snapshot.c

@@ -678,7 +678,9 @@ int bdrv_all_create_snapshot(QEMUSnapshotInfo *sn,
     return 0;
 }
 
-BlockDriverState *bdrv_all_find_vmstate_bs(bool has_devices, strList *devices,
+
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           bool has_devices, strList *devices,
                                            Error **errp)
 {
     g_autoptr(GList) bdrvs = NULL;
@@ -699,13 +701,31 @@ BlockDriverState *bdrv_all_find_vmstate_bs(bool has_devices, strList *devices,
             bdrv_can_snapshot(bs);
         aio_context_release(ctx);
 
-        if (found) {
+        if (vmstate_bs) {
+            if (g_str_equal(vmstate_bs,
+                            bdrv_get_node_name(bs))) {
+                if (found) {
+                    return bs;
+                } else {
+                    error_setg(errp,
+                               "vmstate block device '%s' does not support snapshots",
+                               vmstate_bs);
+                    return NULL;
+                }
+            }
+        } else if (found) {
             return bs;
         }
 
         iterbdrvs = iterbdrvs->next;
     }
 
-    error_setg(errp, "No block device supports snapshots");
+    if (vmstate_bs) {
+        error_setg(errp,
+                   "vmstate block device '%s' does not exist", vmstate_bs);
+    } else {
+        error_setg(errp,
+                   "no block device can store vmstate for snapshot");
+    }
     return NULL;
 }

include/block/snapshot.h

@@ -95,7 +95,8 @@ int bdrv_all_create_snapshot(QEMUSnapshotInfo *sn,
                              strList *devices,
                              Error **errp);
 
-BlockDriverState *bdrv_all_find_vmstate_bs(bool has_devices, strList *devices,
+BlockDriverState *bdrv_all_find_vmstate_bs(const char *vmstate_bs,
+                                           bool has_devices, strList *devices,
                                            Error **errp);
 
 #endif

migration/savevm.c

@@ -2797,7 +2797,7 @@ bool save_snapshot(const char *name, Error **errp)
         }
     }
 
-    bs = bdrv_all_find_vmstate_bs(false, NULL, errp);
+    bs = bdrv_all_find_vmstate_bs(NULL, false, NULL, errp);
     if (bs == NULL) {
         return false;
     }
@@ -2982,7 +2982,7 @@ bool load_snapshot(const char *name, Error **errp)
         return false;
     }
 
-    bs_vm_state = bdrv_all_find_vmstate_bs(false, NULL, errp);
+    bs_vm_state = bdrv_all_find_vmstate_bs(NULL, false, NULL, errp);
     if (!bs_vm_state) {
         return false;
     }

replay/replay-debugging.c

@@ -148,7 +148,7 @@ static char *replay_find_nearest_snapshot(int64_t icount,
 
     *snapshot_icount = -1;
 
-    bs = bdrv_all_find_vmstate_bs(false, NULL, NULL);
+    bs = bdrv_all_find_vmstate_bs(NULL, false, NULL, NULL);
     if (!bs) {
         goto fail;
     }

tests/qemu-iotests/267.out

@@ -6,11 +6,11 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
 Testing:
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
-Error: No block device supports snapshots
+Error: no block device can store vmstate for snapshot
 (qemu) info snapshots
-No block device supports snapshots
+no block device can store vmstate for snapshot
 (qemu) loadvm snap0
-Error: No block device supports snapshots
+Error: no block device can store vmstate for snapshot
 (qemu) quit
 
 
@@ -22,7 +22,7 @@ QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
 Error: Device 'none0' is writable but does not support snapshots
 (qemu) info snapshots
-No block device supports snapshots
+no block device can store vmstate for snapshot
 (qemu) loadvm snap0
 Error: Device 'none0' is writable but does not support snapshots
 (qemu) quit
@@ -58,7 +58,7 @@ QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
 Error: Device 'virtio0' is writable but does not support snapshots
 (qemu) info snapshots
-No block device supports snapshots
+no block device can store vmstate for snapshot
 (qemu) loadvm snap0
 Error: Device 'virtio0' is writable but does not support snapshots
 (qemu) quit
@@ -83,7 +83,7 @@ QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
 Error: Device 'file' is writable but does not support snapshots
 (qemu) info snapshots
-No block device supports snapshots
+no block device can store vmstate for snapshot
 (qemu) loadvm snap0
 Error: Device 'file' is writable but does not support snapshots
 (qemu) quit