Am Do., 26. März 2026 um 22:05 Uhr schrieb Peter Xu <peterx@redhat.com>:
>
> QEMU has a trick in vmstate_save_vmsd_v(), where it will try to compress
> multiple JSON entries into one with a count to avoid duplicated entries.
>
> That only applies to the cases where vmsd_can_compress() should return
> true. For example, vmsd_desc_field_start() later (who will take the
> updated max_elems as the last parameter) will ignore the value passed in
> when vmsd_can_compress() returns false.
>
> Do that check once at the start of loop, and use it to update max_elems, so
> that max_elems keeps 1 for uncompressable VMSD fields, which is more
> straightforward.
>
> This also paves way to make this counter work for ptr marker VMSD fields
> too.
>
> No functional change intended in this patch alone.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
> ---
> migration/vmstate.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/migration/vmstate.c b/migration/vmstate.c
> index e29a8c3f49..05badef42f 100644
> --- a/migration/vmstate.c
> +++ b/migration/vmstate.c
> @@ -556,7 +556,8 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
> void *curr_elem = first_elem + size * i;
> const VMStateField *inner_field;
> bool is_null;
> - int max_elems = n_elems - i;
> + /* maximum number of elements to compress in the JSON blob */
> + int max_elems = vmsd_can_compress(field) ? (n_elems - i) : 1;
>
> old_offset = qemu_file_transferred(f);
> if (field->flags & VMS_ARRAY_OF_POINTER) {
> @@ -587,7 +588,8 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
> * vs. nullptr). Search ahead for the next null/non-null element
> * and start a new compressed array if found.
> */
> - if (vmdesc && (field->flags & VMS_ARRAY_OF_POINTER) &&
> + if (vmdesc && max_elems > 1 &&
> + (field->flags & VMS_ARRAY_OF_POINTER) &&
> is_null != is_prev_null) {
>
> is_prev_null = is_null;
> @@ -626,7 +628,7 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd,
> }
>
> /* Compressed arrays only care about the first element */
> - if (vmdesc_loop && vmsd_can_compress(field)) {
> + if (vmdesc_loop && max_elems > 1) {
> vmdesc_loop = NULL;
> }
> }
> --
> 2.50.1
>