Prasad Pandit <ppan...@redhat.com> writes:
> From: Peter Xu <pet...@redhat.com>
>
> Implement save_postcopy_prepare(), preparing for the enablement of both
> multifd and postcopy.
>
> Please see the rich comment for the rationals.
>
> Signed-off-by: Peter Xu <pet...@redhat.com>
> Signed-off-by: Prasad Pandit <p...@fedoraproject.org>
> ---
> migration/ram.c | 37 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 37 insertions(+)
>
> v8:
> - New patch
>
> v7:
> -
> https://lore.kernel.org/qemu-devel/20250228121749.553184-1-ppan...@redhat.com/T/#t
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 6fd88cbf2a..04fde7ba6b 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -4419,6 +4419,42 @@ 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 to be flushed.
s/to be/have been/
> + *
> + * Meanwhile, the load of these pages must happen before switching
s/Meanwhile,//
> + * 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 destination side.
> + *
> + * This flush and sync will guarantee those pages loaded _before_
s/loaded/are loaded/
> + * postcopy starts on destination. The rational is, this happens
s/rational/rationale/
> + * before VM stops (and before source QEMU sends all the rest of
> + * the postcopy messages). So when the destination QEMU received
> + * 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 should have guaranteed all
> + * previous guest pages queued in the multifd channels to be
> + * 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);
> @@ -4438,6 +4474,7 @@ 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,
