On 2018-11-27 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.
>
> 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)
>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> ---
> hw/s390x/tod-kvm.c | 88 +++++++++++++++++++++++++++++++++++++++++-
> include/hw/s390x/tod.h | 7 +++-
> 2 files changed, 92 insertions(+), 3 deletions(-)
[...]
> @@ -49,10 +112,31 @@ static void kvm_s390_tod_class_init(ObjectClass *oc,
> void *data)
> tdc->set = kvm_s390_tod_set;
> }
>
> +static void kvm_s390_tod_init(Object *obj)
> +{
> + S390TODState *td = S390_TOD(obj);
> +
> + /*
> + * The TOD is initially running (value stored in KVM). Avoid needless
> + * loading/storing of the TOD when starting a simple VM, so let it
> + * run although the (never started) VM is stopped. For migration, we
> + * will properly set the TOD later.
> + */
> + td->stopped = false;
> +
> + /*
> + * We need to know when the VM gets started/stopped to start/stop the
> TOD.
> + * As we can never have more than one TOD instance (and that will never
> be
> + * removed), registering here and never unregistering is good enough.
> + */
> + qemu_add_vm_change_state_handler(kvm_s390_tod_vm_state_change, td);
I think you should rather do this in a realize() function instead, since
instance_init can be called multiple times (during introspection of the
object, for example). See my blog article here for some details:
https://people.redhat.com/~thuth/blog/qemu/2018/09/10/instance-init-realize.html
> +}
> +
> static TypeInfo kvm_s390_tod_info = {
> .name = TYPE_KVM_S390_TOD,
> .parent = TYPE_S390_TOD,
> .instance_size = sizeof(S390TODState),
> + .instance_init = kvm_s390_tod_init,
> .class_init = kvm_s390_tod_class_init,
> .class_size = sizeof(S390TODClass),
> };
Thomas
