linux-kernel - Re: [PATCH 18/25] KVM: TDX: Do TDX specific vcpu initialization

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Message-ID: <1cb07b5d-1e42-44a4-bc0f-adc03433eb90@intel.com>
Date: Mon, 21 Oct 2024 16:35:23 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "Hunter, Adrian" <adrian.hunter@...el.com>,
 "yuan.yao@...ux.intel.com" <yuan.yao@...ux.intel.com>
Cc: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
 <kai.huang@...el.com>, "isaku.yamahata@...il.com"
 <isaku.yamahata@...il.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>,
 "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 18/25] KVM: TDX: Do TDX specific vcpu initialization

On 10/18/2024 10:20 PM, Edgecombe, Rick P wrote:
> On Fri, 2024-10-18 at 10:21 +0800, Xiaoyao Li wrote:
>>> KVM usually leaves it up to userspace to not create nonsensical VMs. So I
>>> think
>>> we can skip the check in KVM.
>>
>> It's not nonsensical unless KVM announces its own requirement for TD
>> guest that userspace VMM must provide valid CPUID leaf 0x1f value for
>> topology.
> 
> How about adding it to the docs?

OK for me.

>>
>> It's architectural valid that userspace VMM creates a TD with legacy
>> topology, i.e., topology enumerated via CPUID 0x1 and 0x4.
>>
>>> In that case, do you see a need for the vanilla tdh_vp_init() SEAMCALL
>>> wrapper?
>>>
>>> The TDX module version we need already supports enum_topology, so the code:
>>>   	if (modinfo->tdx_features0 & MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM)
>>>   		err = tdh_vp_init_apicid(tdx, vcpu_rcx, vcpu->vcpu_id);
>>>   	else
>>>   		err = tdh_vp_init(tdx, vcpu_rcx);
>>>
>>> The tdh_vp_init() branch shouldn't be hit.
>>
>> We cannot know what version of TDX module user might use thus we cannot
>> assume enum_topology is always there unless we make it a hard
>> requirement in KVM that TDX fails being enabled when
>>
>>     !(modinfo->tdx_features0 & MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM)
> 
> We will depend on bugs that are fixed in TDX Modules after enum topology, so it
> shouldn't be required in the normal case. So I think it would be simpler to add
> this tdx_features0 conditional. We can then export one less SEAMCALL and will
> have less configurations flows to worry about on the KVM side.

I'm a little bit confused. what does "add this tdx_feature0 conditional" 
mean?