Quoting Nico Boehr (2022-08-25 13:50:15)
> When running under PV, the guest's TOD clock is under control of the
> ultravisor and the hypervisor isn't allowed to change it. Hence, don't
> allow userspace to change the guest's TOD clock by returning
> -EOPNOTSUPP.
>
> When userspace changes the guest's TOD clock, KVM updates its
> kvm.arch.epoch field and, in addition, the epoch field in all state
> descriptions of all VCPUs.
>
> But, under PV, the ultravisor will ignore the epoch field in the state
> description and simply overwrite it on next SIE exit with the actual
> guest epoch. This leads to KVM having an incorrect view of the guest's
> TOD clock: it has updated its internal kvm.arch.epoch field, but the
> ultravisor ignores the field in the state description.
>
> Whenever a guest is now waiting for a clock comparator, KVM will
> incorrectly calculate the time when the guest should wake up, possibly
> causing the guest to sleep for much longer than expected.
>
> Fixes: 0f3035047140 ("KVM: s390: protvirt: Do only reset registers that are accessible")
> Signed-off-by: Nico Boehr <nrb@xxxxxxxxxxxxx>
This patch seems to break migration (QEMU gets stuck). Possibly a locking issue, I will investigate.
