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vmstate: Update max_elems early and check field compressable once

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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>
This commit is contained in:
Peter Xu
2026-04-01 16:28:37 -04:00
committed by Fabiano Rosas
parent 5aa4714e9d
commit 29439dff51
1 changed files with 5 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
migration/vmstate.c
View File

@@ -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;
}
}