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migration/ram: Implement save_postcopy_prepare()

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Implement save_postcopy_prepare(), preparing for the enablement
of both multifd and postcopy.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250411114534.3370816-5-ppandit@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
This commit is contained in:
Peter Xu 2025-04-11 17:15:31 +05:30
parent 1d48111601
commit ad8d82ffbb
1 changed files with 37 additions and 0 deletions
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37
migration/ram.c
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@ -4398,6 +4398,42 @@ static int ram_resume_prepare(MigrationState *s, void *opaque)
return 0;
}
static bool ram_save_postcopy_prepare(QEMUFile *f, void *opaque, Error **errp)
{
int ret;
if (migrate_multifd()) {
/*
* When multifd is enabled, source QEMU needs to make sure all the
* pages queued before postcopy starts have been flushed.
*
* The load of these pages must happen before switching to postcopy.
* It's because loading of guest pages (so far) in multifd recv
* threads is still non-atomic, so the load cannot happen with vCPUs
* running on the destination side.
*
* This flush and sync will guarantee that those pages are loaded
* _before_ postcopy starts on the destination. The rationale is,
* this happens before VM stops (and before source QEMU sends all
* the rest of the postcopy messages). So when the destination QEMU
* receives the postcopy messages, it must have received the sync
* message on the main channel (either RAM_SAVE_FLAG_MULTIFD_FLUSH,
* or RAM_SAVE_FLAG_EOS), and such message would guarantee that
* all previous guest pages queued in the multifd channels are
* completely loaded.
*/
ret = multifd_ram_flush_and_sync(f);
if (ret < 0) {
error_setg(errp, "%s: multifd flush and sync failed", __func__);
return false;
}
}
qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
return true;
}
void postcopy_preempt_shutdown_file(MigrationState *s)
{
qemu_put_be64(s->postcopy_qemufile_src, RAM_SAVE_FLAG_EOS);
@ -4417,6 +4453,7 @@ static SaveVMHandlers savevm_ram_handlers = {
.load_setup = ram_load_setup,
.load_cleanup = ram_load_cleanup,
.resume_prepare = ram_resume_prepare,
.save_postcopy_prepare = ram_save_postcopy_prepare,
};
static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,