Re: [PATCH RFC] KVM: x86: Drop arbitraty KVM_SOFT_MAX_VCPUS

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/11/21 14:47, Vitaly Kuznetsov wrote:
> KVM_CAP_NR_VCPUS is used to get the "recommended" maximum number of
> VCPUs and arm64/mips/riscv report num_online_cpus(). Powerpc reports
> either num_online_cpus() or num_present_cpus(), s390 has multiple
> constants depending on hardware features. On x86, KVM reports an
> arbitrary value of '710' which is supposed to be the maximum tested
> value but it's possible to test all KVM_MAX_VCPUS even when there are
> less physical CPUs available.
> 
> Drop the arbitrary '710' value and return num_online_cpus() on x86 as
> well. The recommendation will match other architectures and will mean
> 'no CPU overcommit'.
> 
> For reference, QEMU only queries KVM_CAP_NR_VCPUS to print a warning
> when the requested vCPU number exceeds it. The static limit of '710'
> is quite weird as smaller systems with just a few physical CPUs should
> certainly "recommend" less.
> 
> Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Yes, this is a good idea.  We cannot move it entirely to common code due 
to POWER's handling of secondary threads in hypervisors; still, this is 
as close as we can get to a common idea of what KVM_CAP_NR_VCPUS means.

Queued, thanks.

Paolo

> ---
>   arch/x86/include/asm/kvm_host.h | 1 -
>   arch/x86/kvm/x86.c              | 2 +-
>   2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 88fce6ab4bbd..0232a00598f2 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -38,7 +38,6 @@
>   #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
>   
>   #define KVM_MAX_VCPUS 1024
> -#define KVM_SOFT_MAX_VCPUS 710
>   
>   /*
>    * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ac83d873d65b..91ef1b872b90 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4137,7 +4137,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   		r = !static_call(kvm_x86_cpu_has_accelerated_tpr)();
>   		break;
>   	case KVM_CAP_NR_VCPUS:
> -		r = KVM_SOFT_MAX_VCPUS;
> +		r = num_online_cpus();
>   		break;
>   	case KVM_CAP_MAX_VCPUS:
>   		r = KVM_MAX_VCPUS;
>