Re: [PATCH v3 0/4] KVM: TDX: Validate directly configurable CPUID bits

[Binbin Wu]

On 9/8/2026 5:42 PM, Artem Bityutskiy wrote:
> Hi Binbin,
> 
> On Thu, 2026-08-27 at 11:18 +0800, Binbin Wu wrote:
>>
>> Specifically, this series builds a KVM-side allowlist of supported TDX
>> directly configurable CPUID bits to:
>>  - Filter KVM_TDX_CAPABILITIES
>>    Replace the hardcoded denylist to only report configurable bits that
>>    KVM explicitly supports.
>>  - Validate KVM_TDX_INIT_VM
>>    Reject any configurable bit that the TDX module allows but KVM does
>>    not support, as well as CPUID entries with an unexpected subleaf.
> 
> Today's denylist only rejects TSX and WAITPKG. Everything else is allowed.
> 
> Obviously, the TDX module allows directly setting virtual CPUID values
> only for a subset of CPUID leaves, not all of them. So "everything else"
> above is that subset minus TSX and WAITPKG.
> 
> My question is: is there a feature in that "everything else" that
> causes host state clobbering today?
> 
> In other words, does this patch only build the infrastructure for
> addressing future clobbering issues, or does it also fix a specific
> bug?

The Denylist works fine today. But the TDX module evolves.
There are new host state clobbering features coming, e.g. FRED.

The Denylist based solution is not a clean solution:
- It couples the feature enabling for normal VMs with TDX tightly.
- Each time a new feature is added in the denylist, it needs to be backported to old KVM versions

> 
> Thanks!