Introduce this new counter to remember the total dirty bytes for the whole
system.  It will be used for query-migrate command to fetch system-wise
remaining data.

A prior attempt was made to not use this counter but query directly from
all the modules in a QMP handler, but it exposed some complexity not only
on migration state machine race conditions (where the query may be invoked
anytime of the state machine), or on locking implications (where some of
the query hooks may take BQL, which is illegal at least in a QMP handler).
For more information, see:

https://lore.kernel.org/r/aeZMtxqrKWAMKzdN@x1.local

This oneliner will resolve everything, except that it is not as accurate.
The hope is it is a worthwhile trade-off solution, after knowing above
challenges.

Now, there is one more reason we should make each invocation of
save_live_iterate() to be lightweight, because this counter will only get
updated once for each loop over all save_live_iterate() hooks when present.
But that's always the goal.

Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20260421202110.306051-14-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration-stats.h | 7 +++++++
 migration/savevm.c          | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/migration/migration-stats.h b/migration/migration-stats.h
index 1775b916df..9f9a8eb9eb 100644
--- a/migration/migration-stats.h
+++ b/migration/migration-stats.h
@@ -36,6 +36,13 @@ typedef struct {
      * best-effort estimation on expected downtime.
      */
     uint64_t dirty_bytes_last_sync;
+    /*
+     * Number of bytes that were reported dirty now.  This is an estimate
+     * value and will be updated every time migration thread queries from
+     * modules in an iteration loop.  It is used to provide best-effort
+     * estimation on total remaining data.
+     */
+    uint64_t dirty_bytes_total;
     /*
      * Number of pages dirtied per second.
      */
diff --git a/migration/savevm.c b/migration/savevm.c
index 39430470aa..d1dd696c17 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1815,6 +1815,13 @@ void qemu_savevm_query_pending(MigPendingData *pending, bool exact)
     pending->total_bytes = pending->precopy_bytes +
         pending->stopcopy_bytes + pending->postcopy_bytes;
 
+    /*
+     * Update system remaining dirty bytes whenever QEMU queries.  It will
+     * make the value to be not as accurate, but should still be pretty
+     * close to reality when this got invoked frequently while iterating.
+     */
+    mig_stats.dirty_bytes_total = pending->total_bytes;
+
     trace_qemu_savevm_query_pending(exact, pending->precopy_bytes,
                                     pending->stopcopy_bytes,
                                     pending->postcopy_bytes,
-- 
2.53.0