qcow2: Allow lazy refcounts to be enabled on the command line

qcow2 images now accept a boolean lazy_refcounts options. Use it like
this:

  -drive file=test.qcow2,lazy_refcounts=on

If the option is specified on the command line, it overrides the default
specified by the qcow2 header flags that were set when creating the
image.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qcow2-cluster.c

@@ -668,7 +668,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     }
 
     /* Update L2 table. */
-    if (s->compatible_features & QCOW2_COMPAT_LAZY_REFCOUNTS) {
+    if (s->use_lazy_refcounts) {
         qcow2_mark_dirty(bs);
     }
     if (qcow2_need_accurate_refcounts(s)) {

block/qcow2.c

@@ -285,11 +285,26 @@ static int qcow2_check(BlockDriverState *bs, BdrvCheckResult *result,
     return ret;
 }
 
+static QemuOptsList qcow2_runtime_opts = {
+    .name = "qcow2",
+    .head = QTAILQ_HEAD_INITIALIZER(qcow2_runtime_opts.head),
+    .desc = {
+        {
+            .name = "lazy_refcounts",
+            .type = QEMU_OPT_BOOL,
+            .help = "Postpone refcount updates",
+        },
+        { /* end of list */ }
+    },
+};
+
 static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
 {
     BDRVQcowState *s = bs->opaque;
     int len, i, ret = 0;
     QCowHeader header;
+    QemuOpts *opts;
+    Error *local_err = NULL;
     uint64_t ext_end;
 
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
@@ -495,6 +510,28 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
         }
     }
 
+    /* Enable lazy_refcounts according to image and command line options */
+    opts = qemu_opts_create_nofail(&qcow2_runtime_opts);
+    qemu_opts_absorb_qdict(opts, options, &local_err);
+    if (error_is_set(&local_err)) {
+        qerror_report_err(local_err);
+        error_free(local_err);
+        ret = -EINVAL;
+        goto fail;
+    }
+
+    s->use_lazy_refcounts = qemu_opt_get_bool(opts, "lazy_refcounts",
+        (s->compatible_features & QCOW2_COMPAT_LAZY_REFCOUNTS));
+
+    qemu_opts_del(opts);
+
+    if (s->use_lazy_refcounts && s->qcow_version < 3) {
+        qerror_report(ERROR_CLASS_GENERIC_ERROR, "Lazy refcounts require "
+            "a qcow2 image with at least qemu 1.1 compatibility level");
+        ret = -EINVAL;
+        goto fail;
+    }
+
 #ifdef DEBUG_ALLOC
     {
         BdrvCheckResult result = {0};

block/qcow2.h

@@ -173,6 +173,7 @@ typedef struct BDRVQcowState {
 
     int flags;
     int qcow_version;
+    bool use_lazy_refcounts;
 
     uint64_t incompatible_features;
     uint64_t compatible_features;