Re: [PATCH 1/3] KVM: s390: Set KVM_MAX_VCPUS to 256

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Wed, 14 May 2025 18:34:49 +0200
Christoph Schlameuss <schlameuss@linux.ibm.com> wrote:

> The s390x architecture allows for 256 vCPUs with a max CPUID of 255.
> The current KVM implementation limits this to 248 when using the
> extended system control area (ESCA). So this correction should not cause
> any real world problems but actually correct the values returned by the
> ioctls:
> 
> * KVM_CAP_NR_VCPUS
> * KVM_CAP_MAX_VCPUS
> * KVM_CAP_MAX_VCPU_ID
> 
> KVM_MAX_VCPUS is also moved to kvm_host_types to allow using this in
> future type definitions.
> 
> Signed-off-by: Christoph Schlameuss <schlameuss@linux.ibm.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> ---
>  arch/s390/include/asm/kvm_host.h       | 2 --
>  arch/s390/include/asm/kvm_host_types.h | 2 ++
>  arch/s390/kvm/kvm-s390.c               | 2 ++
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index cb89e54ada257eb4fdfe840ff37b2ea639c2d1cb..f51bac835260f562eaf4bbfd373a24bfdbc43834 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -27,8 +27,6 @@
>  #include <asm/isc.h>
>  #include <asm/guarded_storage.h>
>  
> -#define KVM_MAX_VCPUS 255
> -
>  #define KVM_INTERNAL_MEM_SLOTS 1
>  
>  /*
> diff --git a/arch/s390/include/asm/kvm_host_types.h b/arch/s390/include/asm/kvm_host_types.h
> index 1394d3fb648f1e46dba2c513ed26e5dfd275fad4..9697db9576f6c39a6689251f85b4b974c344769a 100644
> --- a/arch/s390/include/asm/kvm_host_types.h
> +++ b/arch/s390/include/asm/kvm_host_types.h
> @@ -6,6 +6,8 @@
>  #include <linux/atomic.h>
>  #include <linux/types.h>
>  
> +#define KVM_MAX_VCPUS 256
> +
>  #define KVM_S390_BSCA_CPU_SLOTS 64
>  #define KVM_S390_ESCA_CPU_SLOTS 248
>  
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 3f3175193fd7a7a26658eb2e2533d8037447a0b4..b65e4cbe67cf70a7d614607ebdd679060e7d31f4 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -638,6 +638,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  			r = KVM_S390_ESCA_CPU_SLOTS;
>  		if (ext == KVM_CAP_NR_VCPUS)
>  			r = min_t(unsigned int, num_online_cpus(), r);
> +		else if (ext == KVM_CAP_MAX_VCPU_ID)
> +			r -= 1;
>  		break;
>  	case KVM_CAP_S390_COW:
>  		r = machine_has_esop();
>