Re: [PATCH v4 24/36] KVM: s390: protvirt: Do only reset registers that are accessible

Christian Borntraeger
2020-02-25T13:07:50Z

On 25.02.20 13:32, Cornelia Huck wrote:
> On Mon, 24 Feb 2020 06:40:55 -0500
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>
>> From: Janosch Frank <frankja@linux.ibm.com>
>>
>> For protected VMs the hypervisor can not access guest breaking event
>> address, program parameter, bpbc and todpr. Do not reset those fields
>> as the control block does not provide access to these fields.
>>
>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>> Reviewed-by: David Hildenbrand <david@redhat.com>
>> [borntraeger@de.ibm.com: patch merging, splitting, fixing]
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>> arch/s390/kvm/kvm-s390.c | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 6ab4c88f2e1d..c734e89235f9 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -3499,14 +3499,16 @@ static void kvm_arch_vcpu_ioctl_initial_reset(struct kvm_vcpu *vcpu)
>> kvm_s390_set_prefix(vcpu, 0);
>> kvm_s390_set_cpu_timer(vcpu, 0);
>> vcpu->arch.sie_block->ckc = 0;
>> - vcpu->arch.sie_block->todpr = 0;
>> memset(vcpu->arch.sie_block->gcr, 0, sizeof(vcpu->arch.sie_block->gcr));
>> vcpu->arch.sie_block->gcr[0] = CR0_INITIAL_MASK;
>> vcpu->arch.sie_block->gcr[14] = CR14_INITIAL_MASK;
>> vcpu->run->s.regs.fpc = 0;
>> - vcpu->arch.sie_block->gbea = 1;
>> - vcpu->arch.sie_block->pp = 0;
>> - vcpu->arch.sie_block->fpf &= ~FPF_BPBC;
>> + if (!kvm_s390_pv_cpu_is_protected(vcpu)) {
>> + vcpu->arch.sie_block->gbea = 1;
>> + vcpu->arch.sie_block->pp = 0;
>> + vcpu->arch.sie_block->fpf &= ~FPF_BPBC;
>> + vcpu->arch.sie_block->todpr = 0;
>
> What happens if we do change those values? Is it just ignored or will
> we get an exception on the next SIE entry?

These fields (offsets in the sie control block) are not defined or re-defined for secure guests.
So we better leave them unchanged.