Re: (Proposal) New TDX Global Metadata To Report FIXED0 and FIXED1 CPUID Bits

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Fri, 2024-12-06 at 10:42 +0800, Xiaoyao Li wrote:
> # Interaction with TDX_FEATURES0.VE_REDUCTION
> 
> TDX introduces a new feature VE_REDUCTION[2]. From the perspective of 
> host VMM, VE_REDUCTION turns several CPUID bits from fixed1 to 
> configurable, e.g., MTRR, MCA, MCE, etc. However, from the perspective 
> of TD guest, it’s an opt-in feature. The actual value seen by TD guest 
> depends on multiple factors: 1). If TD guest enables REDUCE_VE in 
> TDCS.TD_CTLS, 2) TDCS.FEATURE_PARAVIRT_CTRL, 3) CPUID value configured 
> by host VMM via TD_PARAMS.CPUID_CONFIG[]. (Please refer to latest TDX 
> 1.5 spec for more details.)
> 
> Since host VMM has no idea on the setting of 1) and 2) when creating the 
> TD. We make the design to treat them as configurable bits and the global 
> metadata interface doesn’t report them as fixed1 bits for simplicity.
> 
> Host VMM must be aware itself that the value of these VE_REDUCTION 
> related CPUID bits might not be what it configures. The actual value 
> seen by TD guest also depends on the guest enabling and configuration of 
> VE_REDUCTION.

As we've been working on this, I've started to wonder whether this is a halfway
solution that is not worth it. Today there are directly configurable bits,
XFAM/attribute controlled bits, other opt-ins (like #VE reduction). And this has
only gotten more complicated as time has gone on.

If we really want to fully solve the problem of userspace understanding which
configurations are possible, the TDX module would almost need to expose some
sort of CPUID logic DSL that could be used to evaluate user configuration.

On the other extreme we could just say, this kind of logic is just going to need
to be hand coded somewhere, like is currently done in the QEMU patches.

The solution in this proposal decreases the work the VMM has to do, but in the
long term won't remove hand coding completely. As long as we are designing
something, what kind of bar should we target?