Re: [PATCH v11 017/113] KVM: Support KVM_CAP_MAX_VCPUS for KVM_ENABLE_CAP

[Huang, Kai]

On Thu, 2023-01-12 at 08:31 -0800, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> TDX attestation includes the maximum number of vcpu that the guest can
> accommodate.  
> 

I don't understand why "attestation" is the reason here.  Let's say TDX is used
w/o attestation, I don't think this patch can be discarded?

IMHO the true reason is TDX has it's own control of maximum number of vcpus,
i.e. asking you to specify the value when creating the TD.  Therefore, the
constant KVM_MAX_VCPUS doesn't work for TDX guest anymore.

 
> 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@xxxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  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: {
> +		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. */

Why?

> +		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:

Also, IIUC this change is made to the generic kvm_main.c, which means other
archs are  affected too.  Is this OK to other archs?  Why such change cannot
TDX-specific (or, at least x86, or vmx specific)?