linux-kernel - Re: [PATCH v11 2/3] KVM: s390: guest support for topology function

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Date:   Mon, 4 Jul 2022 15:51:48 +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 v11 2/3] KVM: s390: guest support for topology function



On 7/4/22 13:17, Janosch Frank wrote:
> On 7/4/22 13:02, Pierre Morel wrote:
>>
>>
>> On 7/4/22 11:08, Janis Schoetterl-Glausch wrote:
>>> On 7/1/22 18:25, 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
>>> s/he/it (twice)
>>>> 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.And the user STSI capability.
>>> Also: supportS.
>>>>
>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>> Reviewed-by: Nico Boehr <nrb@...ux.ibm.com>
>>>> ---
>>>>    arch/s390/include/asm/kvm_host.h | 18 +++++++++++++---
>>>>    arch/s390/kvm/kvm-s390.c         | 36 
>>>> ++++++++++++++++++++++++++++++++
>>>>    arch/s390/kvm/priv.c             | 16 ++++++++++----
>>>>    arch/s390/kvm/vsie.c             |  8 +++++++
>>>>    4 files changed, 71 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/arch/s390/include/asm/kvm_host.h 
>>>> b/arch/s390/include/asm/kvm_host.h
>>>> index 766028d54a3e..ae6bd3d607de 100644
>>>> --- a/arch/s390/include/asm/kvm_host.h
>>>> +++ b/arch/s390/include/asm/kvm_host.h
>>>> @@ -93,19 +93,30 @@ union ipte_control {
>>>>        };
>>>>    };
>>> [...]
>>>
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index 8fcb56141689..ee59b03f2e45 100644
>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>> @@ -1691,6 +1691,31 @@ 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)
>>>> +{
>>>> +    struct bsca_block *sca = kvm->arch.sca;
>>>> +    union sca_utility new, old;
>>>> +
>>>> +    read_lock(&kvm->arch.sca_lock);
>>>
>>> You forgot to put the assignment of sca under the lock.
>>
>> Should I really?
>> What we want to protect here is the content of the sca.
>> The sca itself does not change during the life of the KVM AFAIK.
> 
> The SCA origin as well as the SCA contents can change within the 
> lifetime of a KVM VM.
> 
> When we switch from bsca to esca we'll use new pages.
> When we add/remove cpus we'll update the MCN and the CPU entry.
> 
> 

Oh! then Yes, right.
thanks


-- 
Pierre Morel
IBM Lab Boeblingen