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

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Date:   Mon, 27 Jun 2022 16:36:47 +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 v10 2/3] KVM: s390: guest support for topology function



On 6/24/22 17:09, 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
>> + * @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)
>> +{
>> +	struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
>> +
>> +	ipte_lock(kvm);
> 
> Why do we need to take the ipte lock here and in patch 3?

That is a good question.
I fear I was tired as I understood that from the documentation, after 
re-reading, we need an interlocked-update not an ipte lock update.
... I have to change that


> 
>> +	sca->utility |= SCA_UTILITY_MTCR;
>> +	ipte_unlock(kvm);
>> +}
> 
> [...]
> 

-- 
Pierre Morel
IBM Lab Boeblingen