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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sun, Jan 15, 2023 at 07:09:03PM +0200, Anton Kuchin wrote:
> Now any vhost-user-fs device makes VM unmigratable, that also prevents
> qemu update without stopping the VM. In most cases that makes sense
> because qemu has no way to transfer FUSE session state.
> 
> But we can give an option to orchestrator to override this if it can
> guarantee that state will be preserved (e.g. it uses migration to
> update qemu and dst will run on the same host as src and use the same
> socket endpoints).
> 
> This patch keeps default behavior that prevents migration with such devices
> but adds migration capability 'vhost-user-fs' to explicitly allow migration.
> 
> Signed-off-by: Anton Kuchin <antonkuchin@yandex-team.ru>
> ---
>  hw/virtio/vhost-user-fs.c | 25 ++++++++++++++++++++++++-
>  qapi/migration.json       |  7 ++++++-
>  2 files changed, 30 insertions(+), 2 deletions(-)

Hi Anton,
Sorry for holding up your work with the discussions that we had. I still
feel it's important to agree on command-line and/or vhost-user protocol
changes that will be able to support non-migratable, stateless
migration/reconnect, and stateful migration vhost-user-fs back-ends. All
three will exist.

As a next step, could you share your code that implements the QEMU side
of stateless migration?

I think that will make it clearer whether a command-line option
(migration capability or per-device) is sufficient or whether the
vhost-user protocol needs to be extended.

If the vhost-user protocol is extended then maybe no user-visible
changes are necessary. QEMU will know if the vhost-user-fs backend
supports migration and which type of migration. It can block migration
in cases where it's not possible.

Thanks,
Stefan

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