Re: [PATCH v4 1/4] KVM: TDX: Track configurable CPUID bits allowed by KVM

[Binbin Wu <binbin.wu@linux.intel.com>]

On 9/23/2026 9:09 AM, Edgecombe, Rick P wrote:
> On Wed, 2026-09-23 at 08:57 +0800, Binbin Wu wrote:
>>> But directly configurable bits don't have any direct conntion to tdcall
>>> queried bits. Userspace can still set those to whatever it wants regardless
>>> of the allow list. 
>>
>> I didn't quite get this.
> 
> VE bits don't query the directly configurable bits in any case unless userspace
> sets it up that way. So they are not involved in the problem we are trying to
> fix here.
> 
>>
>>>
>>> So the argument is basically there is not expected to be any reason to allow
>>> them, so save the code in the allow list. It's a "there is no point" reason.
>>> Makes sense, but I couldn't get that from explanation.
>>
>> I think "not just save the code"?
>> One argument is that if these bits are reported as allowed to userspace and
>> userspace enabled them during init, and the guest doesn't opt-in the #VE
>> reduction (KVM doesn't know the real setting), it would cause problem in the
>> guest.
> 
> Userspace is allowed to cause problems to the guest. We shouldn't try to prevent
> it. 

But userspace doesn't know these bits are not supported by TDX module and KVM.
If KVM report them as supported, and it causes problems, we cannot say it's
userspace's fault.

> But here, I think we are saving lines of allowlist in preventing it. So the
> reason is to have less allow list lines, not to help userspace. And that is
> worth it IMO.