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

Pierre Morel
2022-07-12T07:40:39Z

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@linux.ibm.com>
>> Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
>
> Reviewed-by: Janis Schoetterl-Glausch <scgl@linux.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.

>
>> + 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