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 16:40:38 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "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 Tue, Feb 28, 2023 at 09:57:50PM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> 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.

I'm not sure if I got your point.
Even without attestation, it's allowed to specify max vcpus.   Not "always".
-- 
Isaku Yamahata <isaku.yamahata@...il.com>