Re: [Qemu-devel] [PATCH 06/10] virtio-ccw: use vmstate way for config migration

[Halil Pasic]

On 05/19/2017 07:28 PM, Dr. David Alan Gilbert wrote:
>> To sum it up although I'm currently leaning towards abandoning my idea
>> of two sections for two devices, I'm not comfortable with making the
>> call myself. I'm hoping for some maintainer guidance (s390x, virtio
>> and migration). 
> <A couple of hours of reading, the CCW and PCI code, chatting with
> dhildenb etc>
> 
> OK, so I think:
>   a) First split the series into two separate series; one that
> VMStatifies the existing stuff without breaking compatibility;
> and one that adds the new stuff.  Lets get the first of those
> going in while we think about the second.
> 
>     a.1) I'd do this with patches that convert one chunk into
>      vmstate and remove the corresponding C code at the same time.
> 
>   b) While the way PCI devices are done is weird, I think it'll
> be a lot simpler if you can stick to a structure that's similar
> to them while diverging.  It's hard enough for those of us
> who don't do Virtio every day to get our minds around virtio-pci
> without it being different for virtio-ccw/css.
> 
>   c) To do (b) I suggest:
>       c.1) you *don't* add a vmsd to your virtio_ccw_device
> 
>       c.2) but *do* add a VMSTATE_CCW_DEVICE to the start of any
>          non-virtio devices you migrate (like each of the PCI devices
>           have)
> 
>       c.3) You can add extra state for CSS to the ->save_extra_state
>            handle on virtio devices or to the config
> 
>   d) vmstatifying the config is OK as well.
> 
> I should say I'm no virtio expert, so if any of that's truly
> mad say so.
> 
> Dave
> 

Agreed (a,b,c,d). Reorganizing my patch set according to a) is
going to require some effort, but it should not be too bad. 
About c.2): I don't think we have any migratable non virtio devices
yet, but I'll keep it in mind.

I do not understand what you mean by c.3) and extra_sate. Maybe
it will settle with time. My idea of extending VirtioCcwDevice
is just adding subsections to it's VMStateDescription. The
call vmstate_save_state(f, &vmstate_virtio_ccw_dev, dev, NULL)
in save_config should take care of compatibility. Maybe some
staring at virtio-pci is going to help, but right now I can't tell
what's the extra_state for, and how/why is it 'extra'.

Thanks for your patience!

Regards,
Halil