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

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Date:   Tue, 28 Jun 2022 10:59:03 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Pierre Morel <pmorel@...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 6/20/22 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

I wonder if there is a better name, kvm_s390_report_topology_change maybe?

> + * @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)
> +{

Do we need a sca_lock read_section here? If we don't why not?
Did not see one up the stack, but I might have overlooked something.

> +	struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
> +
> +	ipte_lock(kvm);
> +	sca->utility |= SCA_UTILITY_MTCR;
> +	ipte_unlock(kvm);
> +}
> +

[...]