Re: [ANNOUNCE] PUCK Notes - 2024.04.03 - TDX Upstreaming Strategy

["Huang, Kai" <kai.huang@intel.com>]

On 10/04/2024 4:18 am, Xiaoyao Li wrote:
> On 4/10/2024 12:13 AM, Xiaoyao Li wrote:
>> On 4/9/2024 11:49 PM, Edgecombe, Rick P wrote:
>>>> I don't want JSON.  I want a data payload that is easily consumable 
>>>> in C code,
>>>> which contains (a) the bits that are fixed and (b) their values.  If 
>>>> a value
>>>> can
>>>> change at runtime, it's not fixed.
>>> Right. The fixed values have to come in a reasonable format from the 
>>> TDX module
>>> at runtime, or require an opt-in for any CPUID bits to change in 
>>> future TDX
>>> modules.
>>
>> I have a thought for current situation that TDX module doesn't report 
>> fixed CPUID bits via SEAMCALL interface but defines them in docs. VMM 
>> (KVM or userspace) can maintain a hardcoded array of fixed CPUID bits 
>> and their values according to TDX docs.  And VMM needs to update the 
>> fixed array by striping out the bits that are reported in 
>> TDSYSINFO.CPUID_CONFIG[], which are configurable.
>>
>> If the newer TDX module changes some fixed bits to configurable bits, 
>> They will show up in TDSYSINFO.CPUID_CONFIG[]. So VMM can update fixed 
>> array correctly.
>>
>> In fact, this is how TDX QEMU series current implements.
>>
>> However, it requires TDX module to follow the rule that if any bit 
>> becomes not fixed, it needs to be reported in TDSYSINFO.CPUID_CONFIG[] 
>> as configurable.
> 
> If TDX module flips the bit between fixed0 and fixed1. It doesn't work 
> neither. :(

Exactly.  I think we need to ask TDX module to report such information 
rather than depending on some JASON file.