On Fri, 30 Nov 2018 10:49:57 +0100 David Hildenbrand <da...@redhat.com> 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 <da...@redhat.com> > --- > > v2 -> v3: > - use device_class_set_parent_realize() to implement a child realize > function > > hw/s390x/tod-kvm.c | 102 ++++++++++++++++++++++++++++++++++++++++- > include/hw/s390x/tod.h | 8 +++- > 2 files changed, 107 insertions(+), 3 deletions(-) Thanks, applied.
