Re: [virtio-dev] Re: [Qemu-devel] [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 15, 2018 at 11:32:42AM +0200, Cornelia Huck wrote:
> On Fri, 15 Jun 2018 05:34:24 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Jun 14, 2018 at 12:02:31PM +0200, Cornelia Huck wrote:
> 
> > > > > I am not all that familiar with how Qemu manages network devices. If we can
> > > > > do all the
> > > > > required management of the primary/standby devices within Qemu, that is
> > > > > definitely a better
> > > > > approach without upper layer involvement.    
> > > > 
> > > > Right. I would imagine in the extreme case the upper layer doesn't
> > > > have to be involved at all if QEMU manages all hot plug/unplug logic.
> > > > The management tool can supply passthrough device and virtio with the
> > > > same group UUID, QEMU auto-manages the presence of the primary, and
> > > > hot plug the device as needed before or after the migration.  
> > > 
> > > I do not really see how you can manage that kind of stuff in QEMU only.  
> > 
> > So right now failover is limited to pci passthrough devices only.
> > The idea is to realize the vfio device but not expose it
> > to guest. Have a separate command to expose it to guest.
> > Hotunplug would also hide it from guest but not unrealize it.
> 
> So, this would not be real hot(un)plug, but 'hide it from the guest',
> right? The concept of "we have it realized in QEMU, but the guest can't
> discover and use it" should be translatable to non-pci as well (at
> least for ccw).
> 
> > 
> > This will help ensure that e.g. on migration failure we can
> > re-expose the device without risk of running out of resources.
> 
> Makes sense.
> 
> Should that 'hidden' state be visible/settable from outside as well
> (e.g. via a property)? I guess yes, so that management software has a
> chance to see whether a device is visible.

Might be handy for debug, but note that since QEMU manages this
state it's transient: can change at any time, so it's kind
of hard for management to rely on it.

> Settable may be useful if we
> find another use case for hiding realized devices.