[Qemu-devel] [PATCH 04/16] block: Allow references for backing files

[Wen Congyang <wency@cn.fujitsu.com>]

Usage:
-drive file=xxx,id=Y, \
-drive file=xxxx,id=X,backing.backing_reference=Y

It will create such backing chain:
               {virtio-blk dev 'Y'}      {virtio-blk dev 'X'}
                         |                         |
                         |                         |
                         v                         v

    [base] <- [mid] <- ( Y )  <----------------- ( X )

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
---
 block.c               | 39 +++++++++++++++++++++++++++++++++++----
 include/block/block.h |  1 +
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/block.c b/block.c
index d02d9e9..f93c50d 100644
--- a/block.c
+++ b/block.c
@@ -1179,6 +1179,7 @@ out:
 }
 
 #define ALLOW_WRITE_BACKING_FILE    "allow-write-backing-file"
+#define BACKING_REFERENCE           "backing_reference"
 static QemuOptsList backing_file_opts = {
     .name = "backing_file",
     .head = QTAILQ_HEAD_INITIALIZER(backing_file_opts.head),
@@ -1188,6 +1189,11 @@ static QemuOptsList backing_file_opts = {
             .type = QEMU_OPT_BOOL,
             .help = "allow write to backing file",
         },
+        {
+            .name = BACKING_REFERENCE,
+            .type = QEMU_OPT_STRING,
+            .help = "reference to the exsiting BDS",
+        },
         { /* end of list */ }
     },
 };
@@ -1204,11 +1210,12 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
 {
     char *backing_filename = g_malloc0(PATH_MAX);
     int ret = 0;
-    BlockDriverState *backing_hd;
+    BlockDriverState *backing_hd = NULL;
     Error *local_err = NULL;
     QemuOpts *opts = NULL;
     bool child_rw = false;
     const BdrvChildRole *child_role = NULL;
+    const char *reference = NULL;
 
     if (bs->backing_hd != NULL) {
         QDECREF(options);
@@ -1231,9 +1238,10 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         goto free_exit;
     }
     child_rw = qemu_opt_get_bool(opts, ALLOW_WRITE_BACKING_FILE, false);
+    reference = qemu_opt_get(opts, BACKING_REFERENCE);
     child_role = child_rw ? &child_backing_rw : &child_backing;
 
-    if (qdict_haskey(options, "file.filename")) {
+    if (qdict_haskey(options, "file.filename") || reference) {
         backing_filename[0] = '\0';
     } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
         QDECREF(options);
@@ -1256,7 +1264,9 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         goto free_exit;
     }
 
-    backing_hd = bdrv_new();
+    if (!reference) {
+        backing_hd = bdrv_new();
+    }
 
     if (bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
         qdict_put(options, "driver", qstring_from_str(bs->backing_format));
@@ -1265,7 +1275,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
     assert(bs->backing_hd == NULL);
     ret = bdrv_open_inherit(&backing_hd,
                             *backing_filename ? backing_filename : NULL,
-                            NULL, options, 0, bs, child_role,
+                            reference, options, 0, bs, child_role,
                             NULL, &local_err);
     if (ret < 0) {
         bdrv_unref(backing_hd);
@@ -1276,6 +1286,19 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         error_free(local_err);
         goto free_exit;
     }
+    if (reference) {
+        if (bdrv_op_is_blocked(backing_hd, BLOCK_OP_TYPE_BACKING_REFERENCE,
+                               errp)) {
+            ret = -EBUSY;
+            goto free_reference_exit;
+        }
+        if (backing_hd->blk && blk_disable_attach_dev(backing_hd->blk)) {
+            error_setg(errp, "backing_hd %s is used by the other device model",
+                       reference);
+            ret = -EBUSY;
+            goto free_reference_exit;
+        }
+    }
 
     bdrv_set_backing_hd(bs, backing_hd);
 
@@ -1283,6 +1306,11 @@ free_exit:
     qemu_opts_del(opts);
     g_free(backing_filename);
     return ret;
+
+free_reference_exit:
+    bdrv_unref(backing_hd);
+    bs->open_flags |= BDRV_O_NO_BACKING;
+    goto free_exit;
 }
 
 /*
@@ -1947,6 +1975,9 @@ void bdrv_close(BlockDriverState *bs)
         if (bs->backing_hd) {
             BlockDriverState *backing_hd = bs->backing_hd;
             bdrv_set_backing_hd(bs, NULL);
+            if (backing_hd->blk) {
+                blk_enable_attach_dev(backing_hd->blk);
+            }
             bdrv_unref(backing_hd);
         }
 
diff --git a/include/block/block.h b/include/block/block.h
index 612dcad..5812716 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -170,6 +170,7 @@ typedef enum BlockOpType {
     BLOCK_OP_TYPE_RESIZE,
     BLOCK_OP_TYPE_STREAM,
     BLOCK_OP_TYPE_REPLACE,
+    BLOCK_OP_TYPE_BACKING_REFERENCE,
     BLOCK_OP_TYPE_MAX,
 } BlockOpType;
 
-- 
2.4.3