linux-kernel - Re: [PATCH v10 2/3] KVM: s390: guest support for topology function

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Date:   Tue, 28 Jun 2022 17:01:10 +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 v10 2/3] KVM: s390: guest support for topology function

On 6/28/22 16:13, Pierre Morel wrote:
> 
> 
> On 6/28/22 14:18, Janis Schoetterl-Glausch wrote:
>> On 6/28/22 12:58, Pierre Morel wrote:
>>>
>>>
>>> On 6/28/22 10:59, Janis Schoetterl-Glausch wrote:
>>>> On 6/20/22 14:54, 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 he uses the PTF with command 2 instruction that the
>>>>> topology changed and that he 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
>>>>> support the CPU Topology facility.
>>>>>
>>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>>> ---
>>>>>    arch/s390/include/asm/kvm_host.h | 11 ++++++++---
>>>>>    arch/s390/kvm/kvm-s390.c         | 27 ++++++++++++++++++++++++++-
>>>>>    arch/s390/kvm/priv.c             | 15 +++++++++++----
>>>>>    arch/s390/kvm/vsie.c             |  3 +++
>>>>>    4 files changed, 48 insertions(+), 8 deletions(-)
>>>>>
>>>> [...]
>>>>
>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>> index 8fcb56141689..95b96019ca8e 100644
>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>> @@ -1691,6 +1691,25 @@ static int kvm_s390_get_cpu_model(struct kvm *kvm, struct kvm_device_attr *attr)
>>>>>        return ret;
>>>>>    }
>>>>>
>>>>> +/**
>>>>> + * kvm_s390_sca_set_mtcr
>>>>
>>>> I wonder if there is a better name, kvm_s390_report_topology_change maybe?
>>>>
>>>>> + * @kvm: guest KVM description
>>>>> + *
>>>>> + * Is only relevant if the topology facility is present,
>>>>> + * the caller should check KVM facility 11
>>>>> + *
>>>>> + * Updates the Multiprocessor Topology-Change-Report to signal
>>>>> + * the guest with a topology change.
>>>>> + */
>>>>> +static void kvm_s390_sca_set_mtcr(struct kvm *kvm)
>>>>> +{
>>>>
>>>> Do we need a sca_lock read_section here? If we don't why not?
>>>> Did not see one up the stack, but I might have overlooked something.
>>>
>>> Yes we do.
>>> As I said about your well justified comment in a previous mail, ipte_lock is not the right thing to use here and I will replace with an inter locked update.
>>
>> Not sure I'm understanding you right, you're saying we need both? i.e.:
>>
>> struct bsca_block *sca;
>>
>> read_lock(&vcpu->kvm->arch.sca_lock);
>> sca = kvm->arch.sca;
>> atomic_or(SCA_UTILITY_MTCR, &sca->utility);
>> read_unlock(&vcpu->kvm->arch.sca_lock);
>>
>> Obviously you would need to change the definition of the utility field and could not use a bit field like Janosch
>> suggested, unless you want to use a cmpxchg loop.
>> It's a bit ugly that utility is a two byte value.
>> Maybe there is a nicer way to set that bit, OR (OI, OIY) seem appropriate, but I don't know if they have a nice
>> abstraction in Linux or if you'd need inline asm.
> 
> I was think to something like this because it is what is used most of the time when a bit is to be change concurrently with firmware.

Ah, ok you want to keep the bitfield. 

[...]
> 
> static void kvm_s390_sca_set_mtcr(struct kvm *kvm, int val)

If you use a bool val you can simply do new.mtcr = val; below.
> {
>         struct bsca_block *sca = kvm->arch.sca;
>         union sca_utility new, old;
> 
>         read_lock(&kvm->arch.sca_lock);

Don't forget to move the sca = kvm->arch.sca; under the lock here.
>         do {
>                 old = READ_ONCE(sca->utility);
>                 new = old;
>                 new.mtcr = val ? 1 : 0;
>         } while (cmpxchg(&sca->utility.val, old.val, new.val) != old.val);
>         read_lock(&kvm->arch.sca_lock);
> }
> 
>>>
>>>>
>>>>> +    struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
>>>>> +
>>>>> +    ipte_lock(kvm);
>>>>> +    sca->utility |= SCA_UTILITY_MTCR;
>>>>> +    ipte_unlock(kvm);
>>>>> +}
>>>>> +
>>>>
>>>> [...]
>>>>
>>>
>>
>