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

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Message-ID: <85d3e0ca-186d-197b-308c-d7629488bb8a@linux.ibm.com>
Date:   Wed, 13 Jul 2022 10:42:54 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        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, 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 7/13/22 10:34, Janosch Frank wrote:
> [...]
>>>>>>     +/**
>>>>>> + * 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.
>>>
>>> I don't get what you mean. Nothing about the types changes if you 
>>> move it before the loop.
>>
>> Yes right, did wrong understand.
>> It is better before.
> With the assignment moved one line up:
> Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>

Thanks

> 
>>
>>>>
>>>>
>>>>>
>>>>>> +        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