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Subject: Re: [PATCH v10 6/7] migration/ram.c: add a function to disable the bulk stage

On 12/03/2018 01:31 PM, Peter Xu wrote:
On Mon, Dec 03, 2018 at 10:18:30AM +0800, Wei Wang wrote:
This patch adds a function to enable a precopy notifier callback outside
the migration subsystem to disable the bulk stage flag. This is needed by
the free page optimization offered by virtio-balloon.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Michael S. Tsirkin <mst@redhat.com>
CC: Peter Xu <peterx@redhat.com>
  include/migration/misc.h | 1 +
  migration/ram.c          | 9 +++++++++
  2 files changed, 10 insertions(+)

diff --git a/include/migration/misc.h b/include/migration/misc.h
index 15f8d00..47e7ff5 100644
--- a/include/migration/misc.h
+++ b/include/migration/misc.h
@@ -37,6 +37,7 @@ void precopy_infrastructure_init(void);
  void precopy_add_notifier(NotifierWithReturn *n);
  void precopy_remove_notifier(NotifierWithReturn *n);
  int precopy_notify(PrecopyNotifyReason reason, Error **errp);
+void precopy_disable_bulk_stage(void);
void ram_mig_init(void);
  void qemu_guest_free_page_hint(void *addr, size_t len);
diff --git a/migration/ram.c b/migration/ram.c
index b90a3f2..739dc97 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -354,6 +354,15 @@ int precopy_notify(PrecopyNotifyReason reason, Error **errp)
      return notifier_with_return_list_notify(&precopy_notifier_list, &pnd);
+void precopy_disable_bulk_stage(void)
+    if (!ram_state) {
+        return;
+    }
+    ram_state->ram_bulk_stage = false;
This one is a bit tricky.  E.g., I think it'll at least affect XBZRLE
and compression somehow.  Since we will have the on-off switch for
balloon free page hinting so the user can at least decide what
features to use, with that I don't see much issue with it so far. But
I'd also like to see how other people see this change.

Yes. I think it would be better that this optimization could co-exist with the
compression feature.

How about adding a new flag "ram_state->free_page_optimization_enabled"?

We will then need a change in migration_bitmap_find_dirty:
 - if (rs->ram_bulk_stage && start > 0) {
+ if (!rs->free_page_optimization_enabled && rs->ram_bulk_stage && start > 0) {
        next = start + 1;
    } else {
        next = find_next_bit(bitmap, size, start);


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