This option tells whether the volume should be partitioned or not. Its
default varies: true for hard disks and false for floppy. Its prime
effect is to prevent a master boot record (MBR) to be initialized.

This is useful as some operating system (QNX, Rtems) don't
recognized FAT mounted disks (especially SD cards) if a MBR is present.

Signed-off-by: Clément Chigot <chigot@adacore.com>
---
 block/vvfat.c        | 19 ++++++++++++++++---
 qapi/block-core.json | 10 +++++++---
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index 814796d918..dd0b3689c1 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -306,7 +306,8 @@ typedef struct BDRVVVFATState {
     array_t fat,directory,mapping;
     char volume_label[11];
 
-    uint32_t offset_to_bootsector; /* 0 for floppy, 0x3f for disk */
+    /* 0x3f for partitioned disk, 0x0 otherwise */
+    uint32_t offset_to_bootsector;
 
     unsigned int cluster_size;
     unsigned int sectors_per_cluster;
@@ -1082,6 +1083,11 @@ static QemuOptsList runtime_opts = {
             .type = QEMU_OPT_BOOL,
             .help = "Make the image writable",
         },
+        {
+            .name = "partitioned",
+            .type = QEMU_OPT_BOOL,
+            .help = "Do not add a Master Boot Record on this disk",
+        },
         { /* end of list */ }
     },
 };
@@ -1138,7 +1144,7 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
 {
     BDRVVVFATState *s = bs->opaque;
     int cyls, heads, secs;
-    bool floppy;
+    bool floppy, partitioned;
     const char *dirname, *label;
     QemuOpts *opts;
     int ret;
@@ -1165,6 +1171,9 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
     s->fat_type = qemu_opt_get_number(opts, "fat-type", 0);
     floppy = qemu_opt_get_bool(opts, "floppy", false);
 
+    /* Hard disk are partitioned by default; floppy aren't.  */
+    partitioned = qemu_opt_get_bool(opts, "partitioned", floppy ? false : true);
+
     memset(s->volume_label, ' ', sizeof(s->volume_label));
     label = qemu_opt_get(opts, "label");
     if (label) {
@@ -1196,7 +1205,6 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
         if (!s->fat_type) {
             s->fat_type = 16;
         }
-        s->offset_to_bootsector = 0x3f;
         cyls = s->fat_type == 12 ? 64 : 1024;
         heads = 16;
         secs = 63;
@@ -1215,6 +1223,10 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
+    /* Reserver space for MBR */
+    if (partitioned) {
+        s->offset_to_bootsector = 0x3f;
+    }
 
     s->bs = bs;
 
@@ -3246,6 +3258,7 @@ static const char *const vvfat_strong_runtime_opts[] = {
     "floppy",
     "label",
     "rw",
+    "partitioned",
 
     NULL
 };
diff --git a/qapi/block-core.json b/qapi/block-core.json
index b82af74256..ca438fba51 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3464,8 +3464,8 @@
 #
 # @fat-type: FAT type: 12, 16 or 32
 #
-# @floppy: whether to export a floppy image (true) or partitioned hard
-#     disk (false; default)
+# @floppy: whether to export a floppy image (true) or hard disk
+#     (false; default)
 #
 # @label: set the volume label, limited to 11 bytes.  FAT16 and FAT32
 #     traditionally have some restrictions on labels, which are
@@ -3474,11 +3474,15 @@
 #
 # @rw: whether to allow write operations (default: false)
 #
+# @partitioned: whether the volume will be partitioned;
+#     (default: true for hard disk, false for floppy)
+#     (since 10.2)
+#
 # Since: 2.9
 ##
 { 'struct': 'BlockdevOptionsVVFAT',
   'data': { 'dir': 'str', '*fat-type': 'int', '*floppy': 'bool',
-            '*label': 'str', '*rw': 'bool' } }
+            '*label': 'str', '*rw': 'bool', '*partitioned': 'bool' } }
 
 ##
 # @BlockdevOptionsGenericFormat:
-- 
2.43.0

On Thu, 27 Nov 2025, Clément Chigot wrote:
> This option tells whether the volume should be partitioned or not. Its
> default varies: true for hard disks and false for floppy. Its prime
> effect is to prevent a master boot record (MBR) to be initialized.
>
> This is useful as some operating system (QNX, Rtems) don't
> recognized FAT mounted disks (especially SD cards) if a MBR is present.
>
> Signed-off-by: Clément Chigot <chigot@adacore.com>
> ---
> block/vvfat.c        | 19 ++++++++++++++++---
> qapi/block-core.json | 10 +++++++---
> 2 files changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/block/vvfat.c b/block/vvfat.c
> index 814796d918..dd0b3689c1 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -306,7 +306,8 @@ typedef struct BDRVVVFATState {
>     array_t fat,directory,mapping;
>     char volume_label[11];
>
> -    uint32_t offset_to_bootsector; /* 0 for floppy, 0x3f for disk */
> +    /* 0x3f for partitioned disk, 0x0 otherwise */
> +    uint32_t offset_to_bootsector;
>
>     unsigned int cluster_size;
>     unsigned int sectors_per_cluster;
> @@ -1082,6 +1083,11 @@ static QemuOptsList runtime_opts = {
>             .type = QEMU_OPT_BOOL,
>             .help = "Make the image writable",
>         },
> +        {
> +            .name = "partitioned",
> +            .type = QEMU_OPT_BOOL,
> +            .help = "Do not add a Master Boot Record on this disk",

Maybe should say "Add MBR to disk" and not "Do not add" as that's what 
true means now.

> +        },
>         { /* end of list */ }
>     },
> };
> @@ -1138,7 +1144,7 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
> {
>     BDRVVVFATState *s = bs->opaque;
>     int cyls, heads, secs;
> -    bool floppy;
> +    bool floppy, partitioned;
>     const char *dirname, *label;
>     QemuOpts *opts;
>     int ret;
> @@ -1165,6 +1171,9 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
>     s->fat_type = qemu_opt_get_number(opts, "fat-type", 0);
>     floppy = qemu_opt_get_bool(opts, "floppy", false);
>
> +    /* Hard disk are partitioned by default; floppy aren't.  */

Singular plural mismatch. Either disks/floppies are/aren't or is/isn't 
when singular.

> +    partitioned = qemu_opt_get_bool(opts, "partitioned", floppy ? false : true);
> +
>     memset(s->volume_label, ' ', sizeof(s->volume_label));
>     label = qemu_opt_get(opts, "label");
>     if (label) {
> @@ -1196,7 +1205,6 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
>         if (!s->fat_type) {
>             s->fat_type = 16;
>         }
> -        s->offset_to_bootsector = 0x3f;
>         cyls = s->fat_type == 12 ? 64 : 1024;
>         heads = 16;
>         secs = 63;
> @@ -1215,6 +1223,10 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
>         goto fail;
>     }
>
> +    /* Reserver space for MBR */
> +    if (partitioned) {
> +        s->offset_to_bootsector = 0x3f;
> +    }
>
>     s->bs = bs;
>
> @@ -3246,6 +3258,7 @@ static const char *const vvfat_strong_runtime_opts[] = {
>     "floppy",
>     "label",
>     "rw",
> +    "partitioned",

Does this also needs to be parsed in vvfat_parse_filename like other 
options seem to be?

>
>     NULL
> };
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index b82af74256..ca438fba51 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3464,8 +3464,8 @@
> #
> # @fat-type: FAT type: 12, 16 or 32
> #
> -# @floppy: whether to export a floppy image (true) or partitioned hard
> -#     disk (false; default)
> +# @floppy: whether to export a floppy image (true) or hard disk
> +#     (false; default)

I was wondering what does this option do now. It seems to restrict size to 
different values, maybe that could be explained here.

Regards,
BALATON Zoltan

> #
> # @label: set the volume label, limited to 11 bytes.  FAT16 and FAT32
> #     traditionally have some restrictions on labels, which are
> @@ -3474,11 +3474,15 @@
> #
> # @rw: whether to allow write operations (default: false)
> #
> +# @partitioned: whether the volume will be partitioned;
> +#     (default: true for hard disk, false for floppy)
> +#     (since 10.2)
> +#
> # Since: 2.9
> ##
> { 'struct': 'BlockdevOptionsVVFAT',
>   'data': { 'dir': 'str', '*fat-type': 'int', '*floppy': 'bool',
> -            '*label': 'str', '*rw': 'bool' } }
> +            '*label': 'str', '*rw': 'bool', '*partitioned': 'bool' } }
>
> ##
> # @BlockdevOptionsGenericFormat:
>