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

[Eduardo Habkost]

On Fri, May 31, 2019 at 04:43:44PM -0400, Michael S. Tsirkin wrote:
> On Fri, May 31, 2019 at 07:45:13PM +0100, Dr. David Alan Gilbert wrote:
> > * Michael S. Tsirkin (address@hidden) 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.
> 
> 
> IIUC that doesn't really work with passthrough
> unless guests support surprise removal.

Why?  For the guest, this is indistinguishable from the unplug
request implemented by this series.

-- 
Eduardo