Re: [PATCH] vhost-user-fs: add capability to allow migration

[Stefan Hajnoczi]

On Fri, Feb 10, 2023 at 05:08:16PM +0100, Juan Quintela wrote:
> Anton Kuchin <antonkuchin@yandex-team.ru> wrote:
> > On 02/02/2023 11:59, Juan Quintela wrote:
> >> Anton Kuchin <antonkuchin@yandex-team.ru> wrote:
> >>> On 01/02/2023 16:26, Juan Quintela wrote:
> >>>> Anton Kuchin <antonkuchin@yandex-team.ru> wrote:
> >>>>> On 19/01/2023 18:02, Stefan Hajnoczi wrote:
> >>>>>> On Thu, 19 Jan 2023 at 10:29, Anton Kuchin 
> >>>>>> <antonkuchin@yandex-team.ru> wrote:
> >>>>>>> On 19/01/2023 16:30, Stefan Hajnoczi wrote:
> >>>>>>>> On Thu, 19 Jan 2023 at 07:43, Anton Kuchin 
> >>>>>>>> <antonkuchin@yandex-team.ru> wrote:
> >>>>>>>>> On 18/01/2023 17:52, Stefan Hajnoczi wrote:
> >>>>>>>>>> On Sun, 15 Jan 2023 at 12:21, Anton Kuchin 
> >>>>>>>>>> <antonkuchin@yandex-team.ru> wrote:
> >>>> Once told that, I think that you are making your live harder in the
> >>>> future when you add the other migratable devices.
> >>>>
> >>>> static const VMStateDescription vuf_vmstate = {
> >>>>       .name = "vhost-user-fs",
> >>>>       .minimum_version_id = 0,
> >>>>       .version_id = 0,
> >>>>       .fields = (VMStateField[]) {
> >>>>           VMSTATE_INT8(migration_type, struct VHostUserFS),
> >>>>           VMSTATE_VIRTIO_DEVICE,
> >>>>           VMSTATE_END_OF_LIST()
> >>>>       },
> >>>>       .pre_save = vhost_user_fs_pre_save,
> >>>>       .subsections = (const VMStateDescription*[]) {
> >>>>           &vmstate_vhost_user_fs_internal_sub,
> >>>>           NULL
> >>>>       }
> >>>> };
> >>>>
> >>>> And you are done.
> >>>>
> >>>> I will propose to use a property to set migration_type, but I didn't
> >>>> want to write the code right now.
> >
> > I have a little problem with implementation here and need an advice:
> >
> > Can we make this property adjustable at runtime after device was realized?
> > There is a statement in qemu wiki [1] that makes me think this is possible
> > but I couldn't find any code for it or example in other devices.
> >> "Properties are, by default, locked while the device is
> >   realized. Exceptions
> >> can be made by the devices themselves in order to implement a way
> >   for a user
> >> to interact with a device while it is realized."
> >
> > Or is your idea just to set this property once at construction and keep it
> > constant for device lifetime?
> >
> > [1] https://wiki.qemu.org/Features/QOM
> 
> I have no clue here.  Markus?  Stefan?

Sorry for the late reply. Yes, QOM properties can be set after realize
(e.g. using the qom-set command).

The set() callback can return an error, so some properties are
implemented to refuse updates when ->realize is true.

Stefan

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