RE: [PATCH v2] migration/postcopy: ensure preempt channel is ready before loading states

[Wang, Wei W]

On Friday, April 5, 2024 11:41 AM, Wang, Wei W wrote:
> 
> Before loading the guest states, ensure that the preempt channel has been
> ready to use, as some of the states (e.g. via virtio_load) might trigger page
> faults that will be handled through the preempt channel. So yield to the main
> thread in the case that the channel create event hasn't been dispatched.
> 
> Originally-by: Lei Wang <lei4.wang@intel.com>
> Link: https://lore.kernel.org/all/9aa5d1be-7801-40dd-83fd-
> f7e041ced249@intel.com/T/
> Suggested-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Lei Wang <lei4.wang@intel.com>
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> ---
>  migration/savevm.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c index
> 388d7af7cd..63f9991a8a 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2342,6 +2342,23 @@ static int
> loadvm_handle_cmd_packaged(MigrationIncomingState *mis)
> 
>      QEMUFile *packf = qemu_file_new_input(QIO_CHANNEL(bioc));
> 
> +    /*
> +     * Before loading the guest states, ensure that the preempt channel has
> +     * been ready to use, as some of the states (e.g. via virtio_load) might
> +     * trigger page faults that will be handled through the preempt channel.
> +     * So yield to the main thread in the case that the channel create event
> +     * hasn't been dispatched.
> +     */
> +    do {
> +        if (!migrate_postcopy_preempt() || !qemu_in_coroutine() ||
> +            mis->postcopy_qemufile_dst) {
> +            break;
> +        }
> +
> +        aio_co_schedule(qemu_get_current_aio_context(),
> qemu_coroutine_self());
> +        qemu_coroutine_yield();
> +    } while (1);
> +
>      ret = qemu_loadvm_state_main(packf, mis);
>      trace_loadvm_handle_cmd_packaged_main(ret);
>      qemu_fclose(packf);
> --
> 2.27.0

Main change from v1 is the drop of the wait on sem.
It's still patched to loadvm_handle_cmd_packaged, as the sem issue
(possible twice wait) isn't there now.