[PATCH QEMU HCI-8.1 v6 27/34] migration/ram: don't start dirty logging in CPR modes

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

In the CPR modes guest RAM is preserved in place, not copied:
migrate_ram_is_ignored() is true for every block.  Yet ram_init_bitmaps()
still starts the global dirty log, so every listener starts tracking
pages nobody will send.  KVM write-protects all guest memory, and VFIO
starts DMA logging on the devices; with cpr-exec that logging is never
stopped before the exec, leaving e.g. the DMA page tracker of an NVIDIA
vGPU running after the update.

Don't start the dirty log for a migration that migrates no RAM, as is
already done for background snapshots.  ram_save_cleanup() only stops
the log if it was started.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 migration/ram.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 399b43dd631..0f61942fd3d 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2810,8 +2810,11 @@ static bool ram_init_bitmaps(RAMState *rs, Error **errp)
 
     WITH_RCU_READ_LOCK_GUARD() {
         ram_list_init_bitmaps();
-        /* We don't use dirty log with background snapshots */
-        if (!migrate_background_snapshot()) {
+        /*
+         * We don't use dirty log with background snapshots, nor in CPR modes,
+         * where no RAM is migrated (see migrate_ram_is_ignored()).
+         */
+        if (!migrate_background_snapshot() && !cpr_incoming_needed(NULL)) {
             ret = memory_global_dirty_log_start(GLOBAL_DIRTY_MIGRATION, errp);
             if (!ret) {
                 goto out_unlock;

-- 
2.47.1