linux-kernel - Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX

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Date:   Thu, 11 Nov 2021 19:36:34 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        Andrew Jones <drjones@...hat.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup.patel@....com>,
        Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>, kvm-ppc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS

Hi Vitaly,

On 2021-11-11 16:27, Vitaly Kuznetsov wrote:
> It doesn't make sense to return the recommended maximum number of
> vCPUs which exceeds the maximum possible number of vCPUs.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/arm64/kvm/arm.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 7838e9fb693e..391dc7a921d5 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -223,7 +223,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, 
> long ext)
>  		r = 1;
>  		break;
>  	case KVM_CAP_NR_VCPUS:
> -		r = num_online_cpus();
> +		if (kvm)
> +			r = min_t(unsigned int, num_online_cpus(),
> +				  kvm->arch.max_vcpus);
> +		else
> +			r = min_t(unsigned int, num_online_cpus(),
> +				  kvm_arm_default_max_vcpus());
>  		break;
>  	case KVM_CAP_MAX_VCPUS:
>  	case KVM_CAP_MAX_VCPU_ID:

This looks odd. This means that depending on the phase userspace is
in while initialising the VM, KVM_CAP_NR_VCPUS can return one thing
or the other.

For example, I create a VM on a 32 CPU system, NR_VCPUS says 32.
I create a GICv2 interrupt controller, it now says 8.

That's a change in behaviour that is visible by userspace, which
I'm keen on avoiding. I'd rather have the kvm and !kvm cases
return the same thing.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...