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Date: Mon, 26 Feb 2024 10:52:59 -0800
From: Isaku Yamahata <isaku.yamahata@linux.intel.com>
To: Binbin Wu <binbin.wu@linux.intel.com>
Cc: Yuan Yao <yuan.yao@linux.intel.com>, isaku.yamahata@intel.com,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	isaku.yamahata@gmail.com, Paolo Bonzini <pbonzini@redhat.com>,
	erdemaktas@google.com, Sean Christopherson <seanjc@google.com>,
	Sagi Shahar <sagis@google.com>, Kai Huang <kai.huang@intel.com>,
	chen.bo@intel.com, hang.yuan@intel.com, tina.zhang@intel.com,
	isaku.yamahata@linux.intel.com
Subject: Re: [PATCH v18 023/121] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend
 specific
Message-ID: <20240226185259.GI177224@ls.amr.corp.intel.com>
References: <cover.1705965634.git.isaku.yamahata@intel.com>
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In-Reply-To: <33ef0842-f91f-4c70-821c-0fa41b1d5e6e@linux.intel.com>

On Sun, Feb 04, 2024 at 10:00:45AM +0800,
Binbin Wu <binbin.wu@linux.intel.com> wrote:

> 
> 
> On 2/1/2024 2:16 PM, Yuan Yao wrote:
> > On Wed, Jan 24, 2024 at 09:17:15AM +0800, Binbin Wu wrote:
> > > 
> > > On 1/23/2024 7:52 AM, isaku.yamahata@intel.com wrote:
> > > > From: Isaku Yamahata <isaku.yamahata@intel.com>
> > > > 
> > > > TDX has its own limitation on the maximum number of vcpus that the guest
> > > > can accommodate.  Allow x86 kvm backend to implement its own KVM_ENABLE_CAP
> > > > handler and implement TDX backend for KVM_CAP_MAX_VCPUS.  user space VMM,
> > > > e.g. qemu, can specify its value instead of KVM_MAX_VCPUS.
> > > For legacy VM, KVM just provides the interface to query the max_vcpus.
> > > Why TD needs to provide a interface for userspace to set the limitation?
> > > What's the scenario?
> > I think the reason is TDH.MNG.INIT needs it:
> > 
> > TD_PARAMS:
> >      MAX_VCPUS:
> >          offset: 16 bytes.
> >          type: Unsigned 16b Integer.
> >          size: 2.
> >          Description: Maximum number of VCPUs.
> Thanks for explanation.
> 
> I am also wondering if this info can be passed via KVM_TDX_INIT_VM.
> Because userspace is allowed to set the value no greater than
> min(KVM_MAX_VCPUS, TDX_MAX_VCPUS), providing the extra cap KVM_CAP_MAX_VCPUS
> doesn't make more restriction comparing to providing it in KVM_TDX_INIT_VM.

It's better for the API to be common, not specific to TDX.  Also I don't want
to play with max_vcpu in multiple places.
-- 
Isaku Yamahata <isaku.yamahata@linux.intel.com>