Copy snapshots out of QCOW2 disk

In order to backup snapshots, created from QCOW2 iamge, we want to copy snapshots out of QCOW2 disk to a seperate storage.
The following patch adds a new option in "qemu-img": qemu-img convert -f qcow2 -O qcow2 -s snapshot_name src_img bck_img.
Right now, it only supports to copy the full snapshot, delta snapshot is on the way.

Changes from V1: all the comments from Kevin are addressed:
Add read-only checking
Fix coding style
Change the name from bdrv_snapshot_load to bdrv_snapshot_load_tmp

Signed-off-by: Disheng Su <edison@cloud.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1899,6 +1899,22 @@ int bdrv_snapshot_list(BlockDriverState *bs,
     return -ENOTSUP;
 }
 
+int bdrv_snapshot_load_tmp(BlockDriverState *bs,
+        const char *snapshot_name)
+{
+    BlockDriver *drv = bs->drv;
+    if (!drv) {
+        return -ENOMEDIUM;
+    }
+    if (!bs->read_only) {
+        return -EINVAL;
+    }
+    if (drv->bdrv_snapshot_load_tmp) {
+        return drv->bdrv_snapshot_load_tmp(bs, snapshot_name);
+    }
+    return -ENOTSUP;
+}
+
 #define NB_SUFFIXES 4
 
 char *get_human_readable_size(char *buf, int buf_size, int64_t size)

block.h

@@ -211,6 +211,8 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
 int bdrv_snapshot_delete(BlockDriverState *bs, const char *snapshot_id);
 int bdrv_snapshot_list(BlockDriverState *bs,
                        QEMUSnapshotInfo **psn_info);
+int bdrv_snapshot_load_tmp(BlockDriverState *bs,
+                           const char *snapshot_name);
 char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
 
 char *get_human_readable_size(char *buf, int buf_size, int64_t size);

block/qcow2-snapshot.c

@@ -418,3 +418,34 @@ int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab)
     return s->nb_snapshots;
 }
 
+int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name)
+{
+    int i, snapshot_index, l1_size2;
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+
+    snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_name);
+    if (snapshot_index < 0) {
+        return -ENOENT;
+    }
+
+    sn = &s->snapshots[snapshot_index];
+    s->l1_size = sn->l1_size;
+    l1_size2 = s->l1_size * sizeof(uint64_t);
+    if (s->l1_table != NULL) {
+        qemu_free(s->l1_table);
+    }
+
+    s->l1_table_offset = sn->l1_table_offset;
+    s->l1_table = qemu_mallocz(align_offset(l1_size2, 512));
+
+    if (bdrv_pread(bs->file, sn->l1_table_offset,
+                   s->l1_table, l1_size2) != l1_size2) {
+        return -1;
+    }
+
+    for(i = 0;i < s->l1_size; i++) {
+        be64_to_cpus(&s->l1_table[i]);
+    }
+    return 0;
+}

block/qcow2.c

@@ -1286,6 +1286,7 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_snapshot_goto     = qcow2_snapshot_goto,
     .bdrv_snapshot_delete   = qcow2_snapshot_delete,
     .bdrv_snapshot_list     = qcow2_snapshot_list,
+    .bdrv_snapshot_load_tmp     = qcow2_snapshot_load_tmp,
     .bdrv_get_info	= qcow_get_info,
 
     .bdrv_save_vmstate    = qcow_save_vmstate,

block/qcow2.h

@@ -211,6 +211,7 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
 int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);
 int qcow2_snapshot_delete(BlockDriverState *bs, const char *snapshot_id);
 int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab);
+int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name);
 
 void qcow2_free_snapshots(BlockDriverState *bs);
 int qcow2_read_snapshots(BlockDriverState *bs);

block_int.h

@@ -93,6 +93,8 @@ struct BlockDriver {
     int (*bdrv_snapshot_delete)(BlockDriverState *bs, const char *snapshot_id);
     int (*bdrv_snapshot_list)(BlockDriverState *bs,
                               QEMUSnapshotInfo **psn_info);
+    int (*bdrv_snapshot_load_tmp)(BlockDriverState *bs,
+                                  const char *snapshot_name);
     int (*bdrv_get_info)(BlockDriverState *bs, BlockDriverInfo *bdi);
 
     int (*bdrv_save_vmstate)(BlockDriverState *bs, const uint8_t *buf,

qemu-img-cmds.hx

@@ -28,9 +28,9 @@ STEXI
 ETEXI
 
 DEF("convert", img_convert,
-    "convert [-c] [-f fmt] [-O output_fmt] [-o options] filename [filename2 [...]] output_filename")
+    "convert [-c] [-f fmt] [-O output_fmt] [-o options] [-s snapshot_name] filename [filename2 [...]] output_filename")
 STEXI
-@item convert [-c] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 ETEXI
 
 DEF("info", img_info,

qemu-img.c

@@ -646,13 +646,14 @@ static int img_convert(int argc, char **argv)
     BlockDriverInfo bdi;
     QEMUOptionParameter *param = NULL, *create_options = NULL;
     char *options = NULL;
+    const char *snapshot_name = NULL;
 
     fmt = NULL;
     out_fmt = "raw";
     out_baseimg = NULL;
     flags = 0;
     for(;;) {
-        c = getopt(argc, argv, "f:O:B:hce6o:");
+        c = getopt(argc, argv, "f:O:B:s:hce6o:");
         if (c == -1)
             break;
         switch(c) {
@@ -680,6 +681,9 @@ static int img_convert(int argc, char **argv)
         case 'o':
             options = optarg;
             break;
+        case 's':
+            snapshot_name = optarg;
+            break;
         }
     }
 
@@ -711,6 +715,19 @@ static int img_convert(int argc, char **argv)
         total_sectors += bs_sectors;
     }
 
+    if (snapshot_name != NULL) {
+        if (bs_n > 1) {
+            error("No support for concatenating multiple snapshot\n");
+            ret = -1;
+            goto out;
+        }
+        if (bdrv_snapshot_load_tmp(bs[0], snapshot_name) < 0) {
+            error("Failed to load snapshot\n");
+            ret = -1;
+            goto out;
+        }
+    }
+
     /* Find driver and parse its options */
     drv = bdrv_find_format(out_fmt);
     if (!drv) {

qemu-img.texi

@@ -77,9 +77,9 @@ it doesn't need to be specified separately in this case.
 
 Commit the changes recorded in @var{filename} in its base image.
 
-@item convert [-c] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 
-Convert the disk image @var{filename} to disk image @var{output_filename}
+Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
 using format @var{output_fmt}. It can be optionally compressed (@code{-c}
 option) or use any format specific options like encryption (@code{-o} option).