Re: [RFC PATCH v4 13/36] i386/tdx: Validate TD attributes

[Gerd Hoffmann]

> Validate TD attributes with tdx_caps that fixed-0 bits must be zero and
> fixed-1 bits must be set.

> -static void setup_td_guest_attributes(X86CPU *x86cpu)
> +static int tdx_validate_attributes(TdxGuest *tdx)
> +{
> +    if (((tdx->attributes & tdx_caps->attrs_fixed0) | 
> tdx_caps->attrs_fixed1) !=
> +        tdx->attributes) {
> +            error_report("Invalid attributes 0x%lx for TDX VM (fixed0 
> 0x%llx, fixed1 0x%llx)",
> +                          tdx->attributes, tdx_caps->attrs_fixed0, 
> tdx_caps->attrs_fixed1);
> +            return -EINVAL;
> +    }

So, how is this supposed to work?  Patch #2 introduces attributes as
user-settable property.  So do users have to manually figure and pass
the correct value, so the check passes?  Specifically the fixed1 check?

I think 'attributes' should not be user-settable in the first place.
Each feature-bit which is actually user-settable (and not already
covered by another option like pmu) should be a separate attribute for
tdx-object.  Then the tdx code can create attributes from hardware
capabilities and user settings.

When user-settable options might not be available depending on hardware
capabilities best practice is to create them as OnOffAuto properties.

  Auto == qemu can pick the value, typical behavior is to enable the
          feature if the hardware supports it.
  On == must enable, if it isn't possible throw an error and exit.
  Off == must disable, if it isn't possible throw an error and exit. 

take care,
  Gerd