Re: [Qemu-devel] [PATCH v1] s390x/tod: properly stop the KVM TOD while the guest is not running

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 27 Nov 2018 13:43:24 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 27.11.2018 12:41, David Hildenbrand wrote:
> > Just like on other architectures, we should stop the clock while the guest
> > is not running. This is already properly done for TCG. Right now, doing an
> > offline migration (stop, migrate, cont) can easily trigger stalls in the
> > guest.
> > 
> > Even doing a
> >     (hmp) stop
> >     ... wait 2 minutes ...
> >     (hmp) cont
> > will already trigger stalls.
> > 
> > So whenever the guest stops, backup the KVM TOD. When continuning to run
> > the guest, restore the KVM TOD.  
> 
> We do a similar thing for managedsave so it probably makes sense to solve
> the stall warnings. Now: At the same time, we actually want to have the guest
> see the real time and maybe even share the TOD clock with the host in some
> way, while at the same time avoid the stall warnings.
> 
> One idea is to signal the guest on migration (and stop start events) and let
> the guest do a time sync with the host.
> In fact, it seems that by architecture we need to signal a special sclp signal 
> anyway if stsi information would change (e.g. on migration). Maybe we can 
> piggyback on that and do some time sync interface in the future.

That sounds interesting.

Out of curiosity, is z/VM already doing something like that when you
migrate Linux guests? Or do they do it differently enough from what
QEMU is doing anyway?