Re: [Qemu-devel] [PATCH v3] s390x/tod: Properly stop the KVM TOD while the guest is not running

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 4 Dec 2018 09:27:21 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 30.11.2018 10:49, 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 continuing to run
> > the guest, restore the KVM TOD.
> > 
> > One special case is starting a simple VM: Reading the TOD from KVM to
> > stop it right away until the guest is actually started means that the
> > time of any simple VM will already differ to the host time. We can
> > simply leave the TOD running and the guest won't be able to recognize
> > it.
> > 
> > For migration, we actually want to keep the TOD stopped until really
> > starting the guest. To be able to catch most errors, we should however
> > try to set the TOD in addition to simply storing it. So we can still
> > catch basic migration problems.
> > 
> > If anything goes wrong while backing up/restoring the TOD, we have to
> > ignore it (but print a warning). This is then basically a fallback to
> > old behavior (TOD remains running).
> > 
> > I tested this very basically with an initrd:
> >     1. Start a simple VM. Observed that the TOD is kept running. Old
> >        behavior.
> >     2. Ordinary live migration. Observed that the TOD is temporarily
> >        stopped on the destination when setting the new value and
> >        correctly started when finally starting the guest.
> >     3. Offline live migration. (stop, migrate, cont). Observed that the
> >        TOD will be stopped on the source with the "stop" command. On the
> >        destination, the TOD is temporarily stopped when setting the new
> >        value and correctly started when finally starting the guest via
> >        "cont".
> >     4. Simple stop/cont correctly stops/starts the TOD. (multiple stops
> >        or conts in a row have no effect, so works as expected)
> > 
> > In the future, we might want to send the guest a special kind of time sync
> > interrupt under some conditions, so it can synchronize its tod to the
> > host tod. This is interesting for migration scenarios but also when we
> > get time sync interrupts ourselves. This however will most probably have
> > to be handled in KVM (e.g. when the tods differ too much) and is not
> > desired e.g. when debugging the guest. (single stepping should not
> > result in permanent time syncs). I consider something like that an add-on
> > on top of this basic "don't break the guest" handling.
> > 
> > Signed-off-by: David Hildenbrand <david@redhat.com>  
> 
> 
> Long time we should really work on getting the guest back in sync with the host
> TOD (e..g on migration) since there are some advanced mechanisms that rely on all
> clocks to be in sync. For example the dasd I/O will also write time stamps
> and in an stp complex (synced time across CECs) this can be useful for "classic"
> mainframe databases and ordering.

I think so. It sounds like a bigger effort, though.

> It is probably the right thing to do as of today as on migration we are also out
> of sync.

Nod.

> 
> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
> 
> Adding Viktor in case he has concerns.

I'll go ahead and queue this now, so I don't forget about it (I plan to
send a pull request as soon as 4.0 is out.)

We can still do further changes on top.