qemu-kvm/kvm-migration-block-Make-late-block-active-the-default.patch

From 5a3017fbfb1081fc0a074ee53e1ad7ba8489c8c1 Mon Sep 17 00:00:00 2001
From: Peter Xu <peterx@redhat.com>
Date: Fri, 6 Dec 2024 18:08:35 -0500
Subject: [PATCH 04/23] migration/block: Make late-block-active the default

RH-Author: Kevin Wolf <kwolf@redhat.com>
RH-MergeRequest: 339: QMP command for block device reactivation after migration
RH-Jira: RHEL-54296 RHEL-78397
RH-Acked-by: Eric Blake <eblake@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Commit: [3/22] 74d95ded9153bf0969ede7d7d2708e9452d7cd11 (kmwolf/centos-qemu-kvm)

Migration capability 'late-block-active' controls when the block drives
will be activated.  If enabled, block drives will only be activated until
VM starts, either src runstate was "live" (RUNNING, or SUSPENDED), or it'll
be postponed until qmp_cont().

Let's do this unconditionally.  There's no harm to delay activation of
block drives.  Meanwhile there's no ABI breakage if dest does it, because
src QEMU has nothing to do with it, so it's no concern on ABI breakage.

IIUC we could avoid introducing this cap when introducing it before, but
now it's still not too late to just always do it.  Cap now prone to
removal, but it'll be for later patches.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241206230838.1111496-4-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit fca9aef1c8d8fc4482cc541638dbfac76dc125d6)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 migration/migration.c | 38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index c7a9e2e026..8a262e01ff 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -735,24 +735,6 @@ static void process_incoming_migration_bh(void *opaque)
 
     trace_vmstate_downtime_checkpoint("dst-precopy-bh-enter");
 
-    /* If capability late_block_activate is set:
-     * Only fire up the block code now if we're going to restart the
-     * VM, else 'cont' will do it.
-     * This causes file locking to happen; so we don't want it to happen
-     * unless we really are starting the VM.
-     */
-    if (!migrate_late_block_activate() ||
-        (autostart && runstate_is_live(migration_get_target_runstate()))) {
-        /* Make sure all file formats throw away their mutable metadata.
-         * If we get an error here, just don't restart the VM yet. */
-        bdrv_activate_all(&local_err);
-        if (local_err) {
-            error_report_err(local_err);
-            local_err = NULL;
-            autostart = false;
-        }
-    }
-
     /*
      * This must happen after all error conditions are dealt with and
      * we're sure the VM is going to be running on this host.
@@ -767,7 +749,25 @@ static void process_incoming_migration_bh(void *opaque)
 
     if (runstate_is_live(migration_get_target_runstate())) {
         if (autostart) {
-            vm_start();
+            /*
+             * Block activation is always delayed until VM starts, either
+             * here (which means we need to start the dest VM right now..),
+             * or until qmp_cont() later.
+             *
+             * We used to have cap 'late-block-activate' but now we do this
+             * unconditionally, as it has no harm but only benefit.  E.g.,
+             * it's not part of migration ABI on the time of disk activation.
+             *
+             * Make sure all file formats throw away their mutable
+             * metadata.  If error, don't restart the VM yet.
+             */
+            bdrv_activate_all(&local_err);
+            if (local_err) {
+                error_report_err(local_err);
+                local_err = NULL;
+            } else {
+                vm_start();
+            }
         } else {
             runstate_set(RUN_STATE_PAUSED);
         }
-- 
2.48.1