Re: [Qemu-devel] [PATCH 3/4] net/virtio: add failover support

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> On Fri, May 31, 2019 at 02:01:54PM -0300, Eduardo Habkost wrote:
> > > Yes. It's just lots of extremely low level interfaces
> > > and all rather pointless.
> > > 
> > > And down the road extensions like surprise removal support will make it
> > > all cleaner and more transparent. Floating things up to libvirt means
> > > all these low level details will require more and more hacks.
> > 
> > Why do you call it pointless?
> 
> We'd need APIs to manipulate device visibility to guest, hotplug
> controller state and separately manipulate the resources allocated. This
> is low level stuff that users really have no idea what to do about.
> Exposing such a level of detail to management is imho pointless.
> We are better off with a high level API, see below.

so I don't know much about vfio; but to me it strikes me that
you wouldn't need that low level detail if we just reworked vfio
to look more like all our other devices; something like:

  -vfiodev  host=02:00.0,id=gpu
  -device vfio-pci,dev=gpu

The 'vfiodev' would own the resources; so to do this trick, the
management layer would:
   hotunplug the vfio-pci
   migrate

if anything went wrong it would
   hotplug the vfio-pci backin

you wouldn't have free'd up any resources because they belonged
to the vfiodev.


> > If we want this to work before
> > surprise removal is implemented, we need to provide an API that
> > works for management software.
> >  Don't we want to make this work
> > without surprise removal too?
> 
> This patchset adds an optional, off by default support for
> migrating guests with an assigned network device.
> If enabled this requires guest to allow migration.
> 
> Of course this can be viewed as a security problem since it allows guest
> to block migration. We can't detect a malicious guest reliably imho.
> What we can do is report to management when guest allows migration.
> Policy such what to do when this does not happen for a while and
> what timeout to set would be up to management.
> 
> The API in question would be a high level one, something
> along the lines of a single "guest allowed migration" event.

This is all fairly normal problems with hot unplugging - that's
already dealt with at higher levels for normal hot unplugging.

The question here is to try to avoid duplicating that fairly
painful process in qemu.

Dave

> 
> -- 
> MST
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK