On Tue, 27 Nov 2018 13:43:24 +0100 Christian Borntraeger <borntrae...@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?
