Re: [PATCH v4 2/4] KVM: TDX: Report CORE_CAPABILITIES as configurable

[Edgecombe, Rick P]

On Thu, 2026-09-17 at 15:25 +0800, Binbin Wu wrote:
> Add CORE_CAPABILITIES (CPUID.0x7.0.EDX[30]) to KVM's allowlist of TDX
> directly configurable CPUID feature bits, even though KVM doesn't support
> MSR_IA32_CORE_CAPS for TDX guests, to accommodate the legacy TDX module
> definition and userspace's stale knowledge of it.
> 
> Older TDX specifications define the CORE_CAPABILITIES CPUID bit as
> fixed-1, so userspace may expect the bit to be enabled for TDs.  #VE
> reduction turns it into a directly configurable bit, so leaving it out of
> the allowlist would make the bit impossible to enable once KVM starts
> validating userspace's CPUID input, i.e. would be a surprising behavior
> change for such userspace.

Since #VE reduction is controlled from the guest side, this is a bit confusing.
Turning on #VE reduction can't change CORE_CAPABILITIES configurability status
because it's too late. You mean that this was changed from fixed-1 to
configurable to support #VE reduction arch? (I'm not sure why though).