• Nico Boehr's avatar
    KVM: s390: pv: don't allow userspace to set the clock under PV · 6973091d
    Nico Boehr authored
    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.
    
    With this change, kvm_s390_set_tod() will now take the kvm->lock to be
    able to call kvm_s390_pv_is_protected(). Since kvm_s390_set_tod_clock()
    also takes kvm->lock, use __kvm_s390_set_tod_clock() instead.
    
    The function kvm_s390_set_tod_clock is now unused, hence remove it.
    Update the documentation to indicate the TOD clock attr calls can now
    return -EOPNOTSUPP.
    
    Fixes: 0f303504 ("KVM: s390: protvirt: Do only reset registers that are accessible")
    Reported-by: default avatarMarc Hartmayer <mhartmay@linux.ibm.com>
    Signed-off-by: default avatarNico Boehr <nrb@linux.ibm.com>
    Reviewed-by: default avatarClaudio Imbrenda <imbrenda@linux.ibm.com>
    Reviewed-by: default avatarJanosch Frank <frankja@linux.ibm.com>
    Link: https://lore.kernel.org/r/20221011160712.928239-2-nrb@linux.ibm.com
    Message-Id: <20221011160712.928239-2-nrb@linux.ibm.com>
    Signed-off-by: default avatarJanosch Frank <frankja@linux.ibm.com>
    6973091d
kvm-s390.c 154 KB