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

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Date:   Tue, 12 Jul 2022 10:50:51 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Pierre Morel <pmorel@...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 7/12/22 09:45, Pierre Morel wrote:
> 
> 
> 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.

I don't get what you mean. Nothing about the types changes if you move it before the loop.
> 
> 
>>
>>> +        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);
>>> +}
>>> +
>> [...]
>>
> 
>