From 29439dff51380fc97ddd565e0f05c8e33b597e95 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Wed, 1 Apr 2026 16:28:37 -0400 Subject: [PATCH] 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 Reviewed-by: Alexander Mikhalitsyn Signed-off-by: Peter Xu Reviewed-by: Juraj Marcin Link: https://lore.kernel.org/qemu-devel/20260401202844.673494-5-peterx@redhat.com Signed-off-by: Fabiano Rosas --- 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; } }