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

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Date:   Tue, 12 Jul 2022 11:21:43 +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 7/12/22 10:50, Janis Schoetterl-Glausch wrote:
> 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.

Yes right, did wrong understand.
It is better before.

>>
>>
>>>
>>>> +        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