linux-kernel - Re: [PATCH v11 017/113] KVM: Support KVM_CAP_MAX_VCPUS for KVM_ENABLE

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Date:   Fri, 13 Jan 2023 14:55:07 +0200
From:   Zhi Wang <zhi.wang.linux@...il.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v11 017/113] KVM: Support KVM_CAP_MAX_VCPUS for
 KVM_ENABLE_CAP

On Thu, 12 Jan 2023 08:31:25 -0800
isaku.yamahata@...el.com wrote:

> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> TDX attestation includes the maximum number of vcpu that the guest can
> accommodate.  For that, the maximum number of vcpu needs to be specified
> instead of constant, KVM_MAX_VCPUS.  Make KVM_ENABLE_CAP support
> KVM_CAP_MAX_VCPUS.
> 
> Suggested-by: Sagi Shahar <sagis@...gle.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  virt/kvm/kvm_main.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a235b628b32f..1cfa7da92ad0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4945,7 +4945,27 @@ static int kvm_vm_ioctl_enable_cap_generic(struct
> kvm *kvm, }
>  
>  		mutex_unlock(&kvm->slots_lock);
> +		return r;
> +	}
> +	case KVM_CAP_MAX_VCPUS: {

Better mention the KVM_CAP_MAX_VCPUS defined in XXX patch in the comments.

> +		int r;
>  
> +		if (cap->flags || cap->args[0] == 0)
> +			return -EINVAL;
> +		if (cap->args[0] >  kvm_vm_ioctl_check_extension(kvm,
> KVM_CAP_MAX_VCPUS))
> +			return -E2BIG;
> +
> +		mutex_lock(&kvm->lock);
> +		/* Only decreasing is allowed. */
> +		if (cap->args[0] > kvm->max_vcpus)
> +			r = -E2BIG;
> +		else if (kvm->created_vcpus)
> +			r = -EBUSY;
> +		else {
> +			kvm->max_vcpus = cap->args[0];
> +			r = 0;
> +		}
> +		mutex_unlock(&kvm->lock);
>  		return r;
>  	}
>  	default: