Re: [PATCH v1 1/2] s390x: KVM: accept STSI for CPU topology information

[David Hildenbrand <david@redhat.com>]

On 15.07.21 11:30, Cornelia Huck wrote:
> On Thu, Jul 15 2021, David Hildenbrand <david@redhat.com> wrote:
> 
>> On 14.07.21 17:25, Pierre Morel wrote:
>>> STSI(15.1.x) gives information on the CPU configuration topology.
>>> Let's accept the interception of STSI with the function code 15 and
>>> let the userland part of the hypervisor handle it.
>>>
>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>> ---
>>>    arch/s390/kvm/priv.c | 11 ++++++++++-
>>>    1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>>> index 9928f785c677..4ab5f8b7780e 100644
>>> --- a/arch/s390/kvm/priv.c
>>> +++ b/arch/s390/kvm/priv.c
>>> @@ -856,7 +856,7 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
>>>    	if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE)
>>>    		return kvm_s390_inject_program_int(vcpu, PGM_PRIVILEGED_OP);
>>>    
>>> -	if (fc > 3) {
>>> +	if (fc > 3 && fc != 15) {
>>>    		kvm_s390_set_psw_cc(vcpu, 3);
>>>    		return 0;
>>>    	}
>>> @@ -893,6 +893,15 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
>>>    			goto out_no_data;
>>>    		handle_stsi_3_2_2(vcpu, (void *) mem);
>>>    		break;
>>> +	case 15:
>>> +		if (sel1 != 1 || sel2 < 2 || sel2 > 6)
>>> +			goto out_no_data;
>>> +		if (vcpu->kvm->arch.user_stsi) {
>>> +			insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);
>>> +			return -EREMOTE;
> 
> This bypasses the trace event further down.
> 
>>> +		}
>>> +		kvm_s390_set_psw_cc(vcpu, 3);
>>> +		return 0;
>>>    	}
>>>    	if (kvm_s390_pv_cpu_is_protected(vcpu)) {
>>>    		memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
>>>
>>
>> 1. Setting GPRS to 0
>>
>> I was wondering why we have the "vcpu->run->s.regs.gprs[0] = 0;"
>> for existing fc 1,2,3 in case we set cc=0.
>>
>> Looking at the doc, all I find is:
>>
>> "CC 0: Requested configuration-level number placed in
>> general register 0 or requested SYSIB informa-
>> tion stored"
>>
>> But I don't find where it states that we are supposed to set
>> general register 0 to 0. Wouldn't we also have to do it for
>> fc=15 or for none?
>>
>> If fc 1,2,3 and 15 are to be handled equally, I suggest the following:
>>
>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>> index 9928f785c677..6eb86fa58b0b 100644
>> --- a/arch/s390/kvm/priv.c
>> +++ b/arch/s390/kvm/priv.c
>> @@ -893,17 +893,23 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
>>                           goto out_no_data;
>>                   handle_stsi_3_2_2(vcpu, (void *) mem);
>>                   break;
>> +       case 15:
>> +               if (sel1 != 1 || sel2 < 2 || sel2 > 6)
>> +                       goto out_no_data;
>> +               break;
>>           }
>> -       if (kvm_s390_pv_cpu_is_protected(vcpu)) {
>> -               memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
>> -                      PAGE_SIZE);
>> -               rc = 0;
>> -       } else {
>> -               rc = write_guest(vcpu, operand2, ar, (void *)mem, PAGE_SIZE);
>> -       }
>> -       if (rc) {
>> -               rc = kvm_s390_inject_prog_cond(vcpu, rc);
>> -               goto out;
>> +       if (mem) {
>> +               if (kvm_s390_pv_cpu_is_protected(vcpu)) {
>> +                       memcpy((void *)sida_origin(vcpu->arch.sie_block),
>> +                              (void *)mem, PAGE_SIZE);
>> +               } else {
>> +                       rc = write_guest(vcpu, operand2, ar, (void *)mem,
>> +                                        PAGE_SIZE);
>> +                       if (rc) {
>> +                               rc = kvm_s390_inject_prog_cond(vcpu, rc);
>> +                               goto out;
>> +                       }
>> +               }
>>           }
>>           if (vcpu->kvm->arch.user_stsi) {
>>                   insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);
> 
> Something like that sounds good, the code is getting a bit convoluted.
> 
>>
>>
>> 2. maximum-MNest facility
>>
>> "
>> 1. If the maximum-MNest facility is installed and
>> selector 2 exceeds the nonzero model-depen-
>> dent maximum-selector-2 value."
>>
>> 2. If the maximum-MNest facility is not installed and
>> selector 2 is not specified as two.
>> "
>>
>> We will we be handling the presence/absence of the maximum-MNest facility
>> (for our guest?) in QEMU, corect?
>>
>> I do wonder if we should just let any fc=15 go to user space let the whole
>> sel1 / sel2 checking be handled there. I don't think it's a fast path after all.
>> But no strong opinion.
> 
> If that makes handling easier, I think it would be a good idea.
> 
>>
>> How do we identify availability of maximum-MNest facility?
>>
>>
>> 3. User space awareness
>>
>> How can user space identify that we actually forward these intercepts?
>> How can it enable them? The old KVM_CAP_S390_USER_STSI capability
>> is not sufficient.
> 
> Why do you think that it is not sufficient? USER_STSI basically says
> "you may get an exit that tells you about a buffer to fill in some more
> data for a stsi command, and we also tell you which call". If userspace
> does not know what to add for a certain call, it is free to just do
> nothing, and if it does not get some calls it would support, that should
> not be a problem, either?

If you migrate your VM from machine a to machine b, from kernel a to 
kernel b, and kernel b does not trigger exits to user space for fc=15, 
how could QEMU spot and catch the different capabilities to make sure 
the guest can continue using the feature?


-- 
Thanks,

David / dhildenb