1 files changed, 40 insertions, 1 deletions

diff --git a/migration/migration.h b/migration/migration.h
index 2da2f8a164..310ae8901b 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -65,6 +65,12 @@ typedef struct {
     bool all_zero;
 } PostcopyTmpPage;
 
+typedef enum {
+    PREEMPT_THREAD_NONE = 0,
+    PREEMPT_THREAD_CREATED,
+    PREEMPT_THREAD_QUIT,
+} PreemptThreadStatus;
+
 /* State for the incoming migration */
 struct MigrationIncomingState {
     QEMUFile *from_src_file;
@@ -124,7 +130,12 @@ struct MigrationIncomingState {
     QemuSemaphore postcopy_qemufile_dst_done;
     /* Postcopy priority thread is used to receive postcopy requested pages */
     QemuThread postcopy_prio_thread;
-    bool postcopy_prio_thread_created;
+    /*
+     * Always set by the main vm load thread only, but can be read by the
+     * postcopy preempt thread.  "volatile" makes sure all reads will be
+     * uptodate across cores.
+     */
+    volatile PreemptThreadStatus preempt_thread_status;
     /*
      * Used to sync between the ram load main thread and the fast ram load
      * thread.  It protects postcopy_qemufile_dst, which is the postcopy
@@ -364,6 +375,34 @@ struct MigrationState {
      * do not trigger spurious decompression errors.
      */
     bool decompress_error_check;
+    /*
+     * This variable only affects behavior when postcopy preempt mode is
+     * enabled.
+     *
+     * When set:
+     *
+     * - postcopy preempt src QEMU instance will generate an EOS message at
+     *   the end of migration to shut the preempt channel on dest side.
+     *
+     * - postcopy preempt channel will be created at the setup phase on src
+         QEMU.
+     *
+     * When clear:
+     *
+     * - postcopy preempt src QEMU instance will _not_ generate an EOS
+     *   message at the end of migration, the dest qemu will shutdown the
+     *   channel itself.
+     *
+     * - postcopy preempt channel will be created at the switching phase
+     *   from precopy -> postcopy (to avoid race condtion of misordered
+     *   creation of channels).
+     *
+     * NOTE: See message-id <ZBoShWArKDPpX/D7@work-vm> on qemu-devel
+     * mailing list for more information on the possible race.  Everyone
+     * should probably just keep this value untouched after set by the
+     * machine type (or the default).
+     */
+    bool preempt_pre_7_2;
 
     /*
      * This decides the size of guest memory chunk that will be used