migration: Remember num of ramblocks to sync during recovery

Instead of only relying on the count of rp_sem, make the counter be part of
RAMState so it can be used in both threads to synchronize on the process.

rp_sem will be further reused in follow up patches, as a way to kick the
main thread, e.g., on recovery failures.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231004220240.167175-7-peterx@redhat.com>

migration/ram.c

@@ -394,6 +394,14 @@ struct RAMState {
     /* Queue of outstanding page requests from the destination */
     QemuMutex src_page_req_mutex;
     QSIMPLEQ_HEAD(, RAMSrcPageRequest) src_page_requests;
+
+    /*
+     * This is only used when postcopy is in recovery phase, to communicate
+     * between the migration thread and the return path thread on dirty
+     * bitmap synchronizations.  This field is unused in other stages of
+     * RAM migration.
+     */
+    unsigned int postcopy_bmap_sync_requested;
 };
 typedef struct RAMState RAMState;
 
@@ -4119,20 +4127,20 @@ static int ram_dirty_bitmap_sync_all(MigrationState *s, RAMState *rs)
 {
     RAMBlock *block;
     QEMUFile *file = s->to_dst_file;
-    int ramblock_count = 0;
 
     trace_ram_dirty_bitmap_sync_start();
 
+    qatomic_set(&rs->postcopy_bmap_sync_requested, 0);
     RAMBLOCK_FOREACH_NOT_IGNORED(block) {
         qemu_savevm_send_recv_bitmap(file, block->idstr);
         trace_ram_dirty_bitmap_request(block->idstr);
-        ramblock_count++;
+        qatomic_inc(&rs->postcopy_bmap_sync_requested);
     }
 
     trace_ram_dirty_bitmap_sync_wait();
 
     /* Wait until all the ramblocks' dirty bitmap synced */
-    while (ramblock_count--) {
+    while (qatomic_read(&rs->postcopy_bmap_sync_requested)) {
         qemu_sem_wait(&s->rp_state.rp_sem);
     }
 
@@ -4159,6 +4167,7 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
     unsigned long *le_bitmap, nbits = block->used_length >> TARGET_PAGE_BITS;
     uint64_t local_size = DIV_ROUND_UP(nbits, 8);
     uint64_t size, end_mark;
+    RAMState *rs = ram_state;
 
     trace_ram_dirty_bitmap_reload_begin(block->idstr);
 
@@ -4225,6 +4234,8 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
     /* We'll recalculate migration_dirty_pages in ram_state_resume_prepare(). */
     trace_ram_dirty_bitmap_reload_complete(block->idstr);
 
+    qatomic_dec(&rs->postcopy_bmap_sync_requested);
+
     /*
      * We succeeded to sync bitmap for current ramblock. If this is
      * the last one to sync, we need to notify the main send thread.