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

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

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>
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.

> +		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);
> +}
> +
[...]