linux-kernel - Re: [PATCH v9 2/3] s390x: KVM: guest support for topology function

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Date:   Fri, 17 Jun 2022 16:49:29 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
        thuth@...hat.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, wintera@...ux.ibm.com, seiden@...ux.ibm.com,
        nrb@...ux.ibm.com
Subject: Re: [PATCH v9 2/3] s390x: KVM: guest support for topology function



On 5/16/22 16:13, Pierre Morel wrote:
> 
> 
> On 5/12/22 11:24, David Hildenbrand wrote:
>> On 06.05.22 11:24, Pierre Morel wrote:
>>> We let the userland hypervisor know if the machine support the CPU
>>> topology facility using a new KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>>>
>>> The PTF instruction will report a topology change if there is any change
>>> with a previous STSI_15_1_2 SYSIB.
>>> Changes inside a STSI_15_1_2 SYSIB occur if CPU bits are set or clear
>>> inside the CPU Topology List Entry CPU mask field, which happens with
>>> changes in CPU polarization, dedication, CPU types and adding or
>>> removing CPUs in a socket.
>>>
>>> The reporting to the guest is done using the Multiprocessor
>>> Topology-Change-Report (MTCR) bit of the utility entry of the guest's
>>> SCA which will be cleared during the interpretation of PTF.
>>>
>>> To check if the topology has been modified we use a new field of the
>>> arch vCPU to save the previous real CPU ID at the end of a schedule
>>> and verify on next schedule that the CPU used is in the same socket.
>>> We do not report polarization, CPU Type or dedication change.
>>>
>>> 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 when userland
>>> support the CPU Topology facility.
>>>
>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>
>> [...]
>>
>>
>>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>>> index 0e8603acc105..d9e16b09c8bf 100644
>>> --- a/arch/s390/kvm/priv.c
>>> +++ b/arch/s390/kvm/priv.c
>>> @@ -874,10 +874,12 @@ 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) {
>>> -        kvm_s390_set_psw_cc(vcpu, 3);
>>> -        return 0;
>>> -    }
>>> +    if (fc > 3 && fc != 15)
>>> +        goto out_no_data;
>>> +
>>> +    /* fc 15 is provided with PTF/CPU topology support */
>>> +    if (fc == 15 && !test_kvm_facility(vcpu->kvm, 11))
>>> +        goto out_no_data;
>>
>>
>> Maybe shorter as
>>
>> if (fc == 15 && !test_kvm_facility(vcpu->kvm, 11))
>>     goto out_no_data;
>> else if (fc > 3)
>>     goto out_no_data;
>>
> 
> yes.

hum, sorry, but no.

when test_kvm_facility(11) is true then !test_kvm_facility(11) is false 
and the first test fails
and the second succeed jumping to out_no_data for fc == 15

I can use what I proposed with a comment to make it better readable.
What about:

         /* Bailout forbidden function codes */
         if (fc > 3 && fc != 15)
                 goto out_no_data;
         /* fc 15 is provided with PTF/CPU topology support */
         if (fc == 15 && !test_kvm_facility(vcpu->kvm, 11))
                 goto out_no_data;


> 
>>
>> Apart from that, LGTM.
>>
> 
> Thanks,
> Pierre
> 

-- 
Pierre Morel
IBM Lab Boeblingen