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

Xiaoyao Li
2024-10-21T08:35:29Z

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?