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

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Date:   Mon, 4 Jul 2022 13:17:53 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Pierre Morel <pmorel@...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 13:02, 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.