[Virtio-fs] [PATCH 2/4] vhost-user: Interface for migration state transfer

[Hanna Czenczek]

On 14.04.23 17:17, Eugenio Perez Martin wrote:
> On Thu, Apr 13, 2023 at 7:55 PM Hanna Czenczek <hreitz at redhat.com> wrote:

[...]

>> Basically, what I’m hearing is that I need to implement a different
>> feature that has no practical impact right now, and also fix bugs around
>> it along the way...
>>
> To fix this properly requires iterative device migration in qemu as
> far as I know, instead of using VMStates [1]. This way the state is
> requested to virtiofsd before the device reset.
>
> What does virtiofsd do when the state is totally sent? Does it keep
> processing requests and generating new state or is only a one shot
> that will suspend the daemon? If it is the second I think it still can
> be done in one shot at the end, always indicating "no more state" at
> save_live_pending and sending all the state at
> save_live_complete_precopy.

This sounds to me as if we should reset all devices during migration, 
and I don’t understand that.  virtiofsd will not immediately process 
requests when the state is sent, because the device is still stopped, 
but when it is re-enabled (e.g. because of a failed migration), it will 
have retained its state and continue processing requests as if nothing 
happened.  A reset would break this and other stateful back-ends, as I 
think Stefan has mentioned somewhere else.

It seems to me as if there are devices that need a reset, and so need 
suspend+resume around it, but I also think there are back-ends that 
don’t, where this would only unnecessarily complicate the back-end 
implementation.

Hanna