v3->v4: --- - squash both commits - update docs (thanks Janosch) - add a comment (thanks Janosch) v2->v3: --- - add commit to remove kvm_s390_set_tod_clock() function (thanks Claudio) v1->v2: --- - fix broken migration due to deadlock After a PV guest in QEMU has been paused and resumed, clock comparator interrupts are delivered to the guest much too late. This is caused by QEMU's tod-kvm device restoring the guest's TOD clock upon guest resume. This is not possible with PV, since the guest's TOD clock is controlled by the ultravisor. Even if not allowed under PV, KVM allowed the respective call from userspace (VM attribute KVM_S390_VM_TOD) and updated its internal data structures on this call. This can make the ultravisor's and KVM's view of the guest TOD clock inconsistent. This in turn can lead to the late delivery of clock comparator interrupts when KVM calculates when to wake the guest. This fixes the kernel portion of the problem by disallowing the vm attr call for the guest TOD clock so userspace cannot mess up KVM's view of the guest TOD. This fix causes an ugly warning in QEMU though, hence another fix is due for QEMU to simply not even attempt to set the guest TOD on resume. Nico Boehr (1): KVM: s390: pv: don't allow userspace to set the clock under PV Documentation/virt/kvm/devices/vm.rst | 3 +++ arch/s390/kvm/kvm-s390.c | 26 +++++++++++++++++--------- arch/s390/kvm/kvm-s390.h | 1 - 3 files changed, 20 insertions(+), 10 deletions(-) -- 2.36.1
