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

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On 2/25/20 1:32 PM, Cornelia Huck wrote:
> On Mon, 24 Feb 2020 06:40:55 -0500
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> 
>> From: Janosch Frank <frankja@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>> [borntraeger@xxxxxxxxxx: patch merging, splitting, fixing]
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> ---
>>  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?

Well, changing gbea is a bad idea because of the sida overlay.
I don't think that any other is checked, but I'd need to look up the
todpr changes to be completely sure.

> 
>> +	}
>>  }
>>  
>>  static void kvm_arch_vcpu_ioctl_clear_reset(struct kvm_vcpu *vcpu)
> 


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