linux-kernel - Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX

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Date: Mon, 25 Mar 2024 14:31:18 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>,
	"Huang, Kai" <kai.huang@...el.com>, 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>, chen.bo@...el.com,
	hang.yuan@...el.com, tina.zhang@...el.com,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend
 specific

On Mon, Mar 25, 2024 at 04:42:36PM +0800,
Binbin Wu <binbin.wu@...ux.intel.com> wrote:

> 
> 
> On 3/23/2024 9:13 AM, Isaku Yamahata wrote:
> > On Fri, Mar 22, 2024 at 12:36:40PM +1300,
> > "Huang, Kai" <kai.huang@...el.com> wrote:
> > 
> > > So how about:
> > Thanks for it. I'll update the commit message with some minor fixes.
> > 
> > > "
> > > TDX has its own mechanism to control the maximum number of VCPUs that the
> > > TDX guest can use.  When creating a TDX guest, the maximum number of vcpus
> > > needs to be passed to the TDX module as part of the measurement of the
> > > guest.
> > > 
> > > Because the value is part of the measurement, thus part of attestation, it
> >                                                                             ^'s
> > > better to allow the userspace to be able to configure it.  E.g. the users
> >                    the userspace to configure it                 ^,
> > > may want to precisely control the maximum number of vcpus their precious VMs
> > > can use.
> > > 
> > > The actual control itself must be done via the TDH.MNG.INIT SEAMCALL itself,
> > > where the number of maximum cpus is an input to the TDX module, but KVM
> > > needs to support the "per-VM number of maximum vcpus" and reflect that in
> >                          per-VM maximum number of vcpus
> > > the KVM_CAP_MAX_VCPUS.
> > > 
> > > Currently, the KVM x86 always reports KVM_MAX_VCPUS for all VMs but doesn't
> > > allow to enable KVM_CAP_MAX_VCPUS to configure the number of maximum vcpus
> >                                                       maximum number of vcpus
> > > on VM-basis.
> > > 
> > > Add "per-VM maximum vcpus" to KVM x86/TDX to accommodate TDX's needs.
> > > 
> > > The userspace-configured value then can be verified when KVM is actually
> >                                               used
> 
> Here, "verified", I think Kai wanted to emphasize that the value of
> max_vcpus passed in via
> KVM_TDX_INIT_VM should be checked against the value configured via
> KVM_CAP_MAX_VCPUS?
> 
> Maybe "verified and used" ?

Ok. I don't have strong opinion here.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>