linux-kernel - Re: [PATCH v12 2/3] KVM: s390: guest support for topology function

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Date:   Tue, 12 Jul 2022 09:45:09 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.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,
        david@...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 v12 2/3] KVM: s390: guest support for topology function



On 7/11/22 14:30, Janis Schoetterl-Glausch wrote:
> On 7/11/22 10:41, Pierre Morel wrote:
>> We report a topology change to the guest for any CPU hotplug.
>>
>> The reporting to the guest is done using the Multiprocessor
>> Topology-Change-Report (MTCR) bit of the utility entry in the guest's
>> SCA which will be cleared during the interpretation of PTF.
>>
>> On every vCPU creation we set the MCTR bit to let the guest know the
>> next time it uses the PTF with command 2 instruction that the
>> topology changed and that it should use the STSI(15.1.x) instruction
>> to get the topology details.
>>
>> 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
>> supports the CPU Topology facility.
>>
>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>> Reviewed-by: Nico Boehr <nrb@...ux.ibm.com>
> 
> Reviewed-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>

Thanks.


> See nit below.
>> ---
>>   arch/s390/include/asm/kvm_host.h | 18 +++++++++++++++---
>>   arch/s390/kvm/kvm-s390.c         | 31 +++++++++++++++++++++++++++++++
>>   arch/s390/kvm/priv.c             | 22 ++++++++++++++++++----
>>   arch/s390/kvm/vsie.c             |  8 ++++++++
>>   4 files changed, 72 insertions(+), 7 deletions(-)
>>
> 
> [...]
> 
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 8fcb56141689..70436bfff53a 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -1691,6 +1691,32 @@ static int kvm_s390_get_cpu_model(struct kvm *kvm, struct kvm_device_attr *attr)
>>   	return ret;
>>   }
>>   
>> +/**
>> + * kvm_s390_update_topology_change_report - update CPU topology change report
>> + * @kvm: guest KVM description
>> + * @val: set or clear the MTCR bit
>> + *
>> + * Updates the Multiprocessor Topology-Change-Report bit to signal
>> + * the guest with a topology change.
>> + * This is only relevant if the topology facility is present.
>> + *
>> + * The SCA version, bsca or esca, doesn't matter as offset is the same.
>> + */
>> +static void kvm_s390_update_topology_change_report(struct kvm *kvm, bool val)
>> +{
>> +	union sca_utility new, old;
>> +	struct bsca_block *sca;
>> +
>> +	read_lock(&kvm->arch.sca_lock);
>> +	do {
>> +		sca = kvm->arch.sca;
> 
> I find this assignment being in the loop unintuitive, but it should not make a difference.

The price would be an ugly cast.


> 
>> +		old = READ_ONCE(sca->utility);
>> +		new = old;
>> +		new.mtcr = val;
>> +	} while (cmpxchg(&sca->utility.val, old.val, new.val) != old.val);
>> +	read_unlock(&kvm->arch.sca_lock);
>> +}
>> +
> [...]
> 


-- 
Pierre Morel
IBM Lab Boeblingen