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

[Wang, Lei]

On 4/6/2024 5:53, Peter Xu wrote:> 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.

I'm OK with it.

> 
> +     * 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!

Thank you Peter :)

> 
>> +     */
>> +    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
>>
>