vmstate: Update max_elems early and check field compressable once

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.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260401202844.673494-5-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

diff --git a/migration/vmstate.c b/migration/vmstate.c
index e29a8c3f4969a4069530d46bde53a34a5956dd96..05badef42fa95c13d868e0f240d2794cb3f0d0dd 100644 (file)
--- 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;
                 }
             }