Other command line options like "-chardev", "-netdev" or "-device"
already present the user with a list of available parameters, but
when running "-blockdev help", there is currently only an error
message. There is already the possibility to run "-drive format=help",
but it does not seem to be documented, so this is not very useful
for the average user. Thus let's improve this situation by providing
a proper list of available drivers when running QEMU with
"-blockdev help".

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v2: Don't re-use the code from "-drive format=help" and print the
     help in a better readable way instead

 include/block/block-common.h |  1 +
 block.c                      | 15 +++++++++++++++
 system/vl.c                  |  7 +++++--
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/block/block-common.h b/include/block/block-common.h
index 895ea175413..c76a72c1220 100644
--- a/include/block/block-common.h
+++ b/include/block/block-common.h
@@ -583,5 +583,6 @@ char *path_combine(const char *base_path, const char *filename);
 char *bdrv_get_full_backing_filename_from_filename(const char *backed,
                                                    const char *backing,
                                                    Error **errp);
+void bdrv_show_help(void);
 
 #endif /* BLOCK_COMMON_H */
diff --git a/block.c b/block.c
index f0a6042e616..895d6db5acf 100644
--- a/block.c
+++ b/block.c
@@ -524,6 +524,21 @@ bool bdrv_uses_whitelist(void)
     return use_bdrv_whitelist;
 }
 
+static void bdrv_help_format_print(void *opaque, const char *name)
+{
+    if (bdrv_format_is_whitelisted(name, false)) {
+        printf("%s\n", name);
+    } else {
+        printf("%s (read-only)\n", name);
+    }
+}
+
+void bdrv_show_help(void)
+{
+      printf("Available blockdev drivers:\n");
+      bdrv_iterate_format(bdrv_help_format_print, NULL, true);
+}
+
 typedef struct CreateCo {
     BlockDriver *drv;
     char *filename;
diff --git a/system/vl.c b/system/vl.c
index 9bd7664b85c..dc93e8737cb 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -2960,7 +2960,10 @@ void qemu_init(int argc, char **argv)
                           HD_OPTS);
                 break;
             case QEMU_OPTION_blockdev:
-                {
+                if (is_help_option(optarg)) {
+                    bdrv_show_help();
+                    exit(EXIT_SUCCESS);
+                } else {
                     Visitor *v;
                     BlockdevOptionsQueueEntry *bdo;
 
@@ -2973,8 +2976,8 @@ void qemu_init(int argc, char **argv)
                     visit_free(v);
                     loc_save(&bdo->loc);
                     QSIMPLEQ_INSERT_TAIL(&bdo_queue, bdo, entry);
-                    break;
                 }
+                break;
             case QEMU_OPTION_drive:
                 if (!qemu_opts_parse_noisily(qemu_find_opts("drive"),
                                              optarg, false)) {
-- 
2.55.0