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

[Juan Quintela]

Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Tue, Jan 24, 2023, 04:50 Dr. David Alan Gilbert <dgilbert@redhat.com>
> wrote:

[...]

>> > I checked how bad the situation is. libvhost-user currently enables
>> > LOG_ALL by default. :(
>> >
>> > So I don't think the front-end can use LOG_ALL alone to determine
>> > whether or not migration is supported by the back-end.
>> >
>> > There are several existing back-ends based on libvhost-user that have
>> > no concept of reconnection or migration but report the LOG_ALL feature
>> > bit.
>>
>> Ouch, yes that's messy.
>>
>> Going back to the original question; I don't think a command line flag
>> will work though, because even for a given VM there's the possibility
>> of some (local) migrations working but other (remote) migrations not
>> working; so you don't know at the point you start the VM whether
>> your migrations are going to work.
>>
>
> The user or management tool should know which types of migration a
> vhost-user-fs backend supports. That can be passed in as a per-device
> parameter.
>
> Then a migration parameter can be used to distinguish between same host and
> remote host migration? QEMU already distinguishes between pre-copy and
> post-copy migration, so this can be thought of as yet another type of
> migration.

I was going to suggest this (my previous answer was after reading only
the other part of the comments).

What we have here is that this device has "three" states:
- You can't migrate it to other host (now and the default behaviour)
- You can migrate some of the backends if you are migrating in the same
  host (note, we don't know directly that we are migrating inside the
  same host, so I would agree to add _that_ migration capability, that
  is related to migration, and it makes sense for migration code and
  devices to know that is happening)
- In the future, perhaps, you can migrate "always" some vhost-use-fs,
  that would be a property on my opinion.

Later, Juan.