Add the 'detached-mode' option to specify the creation of
a detached LUKS header. This is how it is used:
$ qemu-img create --object secret,id=sec0,data=abc123 -f luks
> -o cipher-alg=aes-256,cipher-mode=xts -o key-secret=sec0
> -o detached-mode=true header.luks

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
---
 block.c          | 5 ++++-
 block/crypto.c   | 9 ++++++++-
 block/crypto.h   | 8 ++++++++
 qapi/crypto.json | 5 ++++-
 4 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index bfb0861ec6..fa9ce36928 100644
--- a/block.c
+++ b/block.c
@@ -7517,7 +7517,10 @@ void bdrv_img_create(const char *filename, const char *fmt,
         goto out;
     }
 
-    if (size == -1) {
+    /* Parameter 'size' is not needed for detached LUKS header */
+    if (size == -1 &&
+        !(!strcmp(fmt, "luks") &&
+          qemu_opt_get_bool(opts, "detached-mode", false))) {
         error_setg(errp, "Image creation needs a size parameter");
         goto out;
     }
diff --git a/block/crypto.c b/block/crypto.c
index 76cc8bda49..812c3c28f5 100644
--- a/block/crypto.c
+++ b/block/crypto.c
@@ -229,6 +229,7 @@ static QemuOptsList block_crypto_create_opts_luks = {
         BLOCK_CRYPTO_OPT_DEF_LUKS_IVGEN_HASH_ALG(""),
         BLOCK_CRYPTO_OPT_DEF_LUKS_HASH_ALG(""),
         BLOCK_CRYPTO_OPT_DEF_LUKS_ITER_TIME(""),
+        BLOCK_CRYPTO_OPT_DEF_LUKS_DETACHED_MODE(""),
         { /* end of list */ }
     },
 };
@@ -793,6 +794,8 @@ block_crypto_co_create_opts_luks(BlockDriver *drv, const char *filename,
     PreallocMode prealloc;
     char *buf = NULL;
     int64_t size;
+    bool detached_mode =
+        qemu_opt_get_bool(opts, "detached-mode", false);
     int ret;
     Error *local_err = NULL;
 
@@ -832,8 +835,12 @@ block_crypto_co_create_opts_luks(BlockDriver *drv, const char *filename,
         goto fail;
     }
 
+   /* The detached_header default to true if detached-mode is specified */
+    create_opts->u.luks.detached_header = detached_mode ? true : false;
+
     /* Create format layer */
-    ret = block_crypto_co_create_generic(bs, size, create_opts, prealloc, errp);
+    ret = block_crypto_co_create_generic(bs, detached_mode ? 0 : size,
+                                         create_opts, prealloc, errp);
     if (ret < 0) {
         goto fail;
     }
diff --git a/block/crypto.h b/block/crypto.h
index 72e792c9af..bceefd45bd 100644
--- a/block/crypto.h
+++ b/block/crypto.h
@@ -41,6 +41,7 @@
 #define BLOCK_CRYPTO_OPT_LUKS_IVGEN_HASH_ALG "ivgen-hash-alg"
 #define BLOCK_CRYPTO_OPT_LUKS_HASH_ALG "hash-alg"
 #define BLOCK_CRYPTO_OPT_LUKS_ITER_TIME "iter-time"
+#define BLOCK_CRYPTO_OPT_LUKS_DETACHED_MODE "detached-mode"
 #define BLOCK_CRYPTO_OPT_LUKS_KEYSLOT "keyslot"
 #define BLOCK_CRYPTO_OPT_LUKS_STATE "state"
 #define BLOCK_CRYPTO_OPT_LUKS_OLD_SECRET "old-secret"
@@ -100,6 +101,13 @@
         .help = "Select new state of affected keyslots (active/inactive)",\
     }
 
+#define BLOCK_CRYPTO_OPT_DEF_LUKS_DETACHED_MODE(prefix)     \
+    {                                                         \
+        .name = prefix BLOCK_CRYPTO_OPT_LUKS_DETACHED_MODE, \
+        .type = QEMU_OPT_BOOL,                                \
+        .help = "Create a detached LUKS header",              \
+    }
+
 #define BLOCK_CRYPTO_OPT_DEF_LUKS_KEYSLOT(prefix)              \
     {                                                          \
         .name = prefix BLOCK_CRYPTO_OPT_LUKS_KEYSLOT,          \
diff --git a/qapi/crypto.json b/qapi/crypto.json
index 6b4e86cb81..8e81aa8454 100644
--- a/qapi/crypto.json
+++ b/qapi/crypto.json
@@ -226,6 +226,8 @@
 # @iter-time: number of milliseconds to spend in PBKDF passphrase
 #     processing.  Currently defaults to 2000. (since 2.8)
 #
+# @detached-mode: create a detached LUKS header. (since 9.0)
+#
 # Since: 2.6
 ##
 { 'struct': 'QCryptoBlockCreateOptionsLUKS',
@@ -235,7 +237,8 @@
             '*ivgen-alg': 'QCryptoIVGenAlgorithm',
             '*ivgen-hash-alg': 'QCryptoHashAlgorithm',
             '*hash-alg': 'QCryptoHashAlgorithm',
-            '*iter-time': 'int'}}
+            '*iter-time': 'int',
+            '*detached-mode': 'bool'}}
 
 ##
 # @QCryptoBlockOpenOptions:
-- 
2.39.1