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

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On 25.02.20 14:06, Cornelia Huck wrote:
> On Tue, 25 Feb 2020 13:51:12 +0100
> Janosch Frank <frankja@xxxxxxxxxxxxx> wrote:
> 
>> 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.
> 
> Maybe add a comment
> 
> /*
>  * Do not reset these registers in the protected case, as some of
>  * them are overlayed and they are not accessible in this case
>  * anyway.
>  */

Makes sense, will add.




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