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

[Peter Xu]

On Fri, Apr 05, 2024 at 11:40:56AM +0800, Wei Wang 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: 
> 9aa5d1be-7801-40dd-83fd-f7e041ced249@intel.com/T/">https://lore.kernel.org/all/9aa5d1be-7801-40dd-83fd-f7e041ced249@intel.com/T/
> Suggested-by: Peter Xu <peterx@redhat.com>

The current version doesn't have any of my credits. :) Thanks, but I'll
just drop it to reflect reality, so we keep the credit to the right ones.

> 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.

I'll squash below into it.  If any of you disagree please shoot: valid
until this Sunday.

+     * TODO: if we can move migration loadvm out of main thread, then we
+     * won't block main thread from polling the accept() fds.  We can drop
+     * this as a whole when that is done.

Huge thanks to both!

> +     */
> +    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
> 

-- 
Peter Xu