[Qemu-devel] [PATCH v3] migratioin/ram: leave RAMBlock->bmap blank on allocating

[Wei Yang]

During migration, we would sync bitmap from ram_list.dirty_memory to
RAMBlock.bmap in cpu_physical_memory_sync_dirty_bitmap().

Since we set RAMBlock.bmap and ram_list.dirty_memory both to all 1, this
means at the first round this sync is meaningless and is a duplicated
work.

Leaving RAMBlock->bmap blank on allocating would have a side effect on
migration_dirty_pages, since it is calculated from the result of
cpu_physical_memory_sync_dirty_bitmap(). To keep it right, we need to
set migration_dirty_pages to 0 in ram_state_init().

Signed-off-by: Wei Yang <address@hidden>
Reviewed-by: Dr. David Alan Gilbert <address@hidden>

---
v3: adjust comment based on Peter's comments
v2: add a comment explaining why leaving RAMBlock.bmap clear
---
 migration/ram.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 4c60869226..c3ece382ae 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -3182,11 +3182,11 @@ static int ram_state_init(RAMState **rsp)
     QSIMPLEQ_INIT(&(*rsp)->src_page_requests);
 
     /*
-     * Count the total number of pages used by ram blocks not including any
-     * gaps due to alignment or unplugs.
+     * This must match with the initial values of dirty bitmap.
+     * Currently we initialize the dirty bitmap to all zeros so
+     * here the total dirty page count is zero.
      */
-    (*rsp)->migration_dirty_pages = ram_bytes_total() >> TARGET_PAGE_BITS;
-
+    (*rsp)->migration_dirty_pages = 0;
     ram_state_reset(*rsp);
 
     return 0;
@@ -3201,8 +3201,16 @@ static void ram_list_init_bitmaps(void)
     if (ram_bytes_total()) {
         RAMBLOCK_FOREACH_NOT_IGNORED(block) {
             pages = block->max_length >> TARGET_PAGE_BITS;
+            /*
+             * The initial dirty bitmap for migration must be set with all
+             * ones to make sure we'll migrate every guest RAM page to
+             * destination.
+             * Here we didn't set RAMBlock.bmap simply because it is already
+             * set in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION] in
+             * ram_block_add, and that's where we'll sync the dirty bitmaps.
+             * Here setting RAMBlock.bmap would be fine too but not necessary.
+             */
             block->bmap = bitmap_new(pages);
-            bitmap_set(block->bmap, 0, pages);
             if (migrate_postcopy_ram()) {
                 block->unsentmap = bitmap_new(pages);
                 bitmap_set(block->unsentmap, 0, pages);
-- 
2.19.1