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

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Date:   Mon, 16 May 2022 16:13:10 +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 5/12/22 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.

> 
> Apart from that, LGTM.
> 

Thanks,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen