Re: [PATCH 1/2] x86/cpuid: Infrastructure to support pseudo feature identifiers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.10.2022 18:08, Andrew Cooper wrote:
> A future change will want a cpuid-like identifier which doesn't have a mapping
> to a feature bit.
> 
>  * Pass the feature name into the parse callback.
>  * Exclude a feature value of ~0u from falling into the general set/clear bit
>    paths.
>  * In gen-cpuid.py, insert a placeholder to collect all the pseudo feature
>    names.

Hmm, I was envisioning this to be fitted into the existing model in a
less adhoc way: (parts of) MSRs holding feature bits aren't very different
from individual (pairs of) registers of CPUID output (in the case of
ARCH_CAPS there would be a perhaps just abstract mask limiting things to
the subset of bits which actually act as feature indicators). Hence I'd
have expected them to gain proper entries in the public interface
(cpufeatureset.h) and then be represented / processed the same way in
featuresets and policies. All that would be left out at this point would
be the exposing of the bit to guests (in patch 2, assuming the split into
two patches is then actually still warranted), integration into
guest_rdmsr(), and at least some of the tool stack side (xen-cpuid, for
example, could easily learn of such right away).

However, since I'm pretty sure you've considered such an approach, I guess
I might be overlooking some caveat?

> --- a/xen/tools/gen-cpuid.py
> +++ b/xen/tools/gen-cpuid.py
> @@ -297,6 +297,19 @@ def crunch_numbers(state):
>          RTM: [TSXLDTRK],
>      }
>  
> +    #
> +    # Pseudo feature names.  These don't map to a feature bit, but are
> +    # inserted into the values dictionary so they can be parsed and handled
> +    # specially
> +    #
> +    pseduo_names = (
> +    )
> +
> +    for n in pseduo_names:
> +        if n in state.values:
> +            raise Fail("Pseduo feature name %s aliases real feature" % (n, ))
> +        state.values[n] = 0xffffffff

Throughout this hunk: s/pseduo/pseudo/g.

Jan