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

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Date:   Tue, 28 Feb 2023 21:57:50 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
CC:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH v11 017/113] KVM: Support KVM_CAP_MAX_VCPUS for
 KVM_ENABLE_CAP

On Mon, 2023-02-27 at 13:26 -0800, Isaku Yamahata wrote:
> > > 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.
> 
> Without TDX attestation, this can be discarded.  The TD is created with
> max_vcpus=KVM_MAX_VCPUS by default.

This parses like: 

If we have attestation, the TD can be created with a user-specified non-default
value.  Otherwise, the TD is always created with default value.

It doesn't make sense, right?

Because architecturally whether TD can be created with a user specified value
doesn't depend on attestation at all.