Re: [PATCH 2/8] x86/cpu-policy: define bits of leaf 6

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/11/2025 3:06 pm, Jan Beulich wrote:
> ... as far as we presently use them in the codebase.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Or should we make both parts proper featureset elements? At least
> APERFMPERF could likely be made visible to guests (in principle).

As before, A/MPERF can't be used safely by a VM.

In order to be persuaded to offer this to VMs, someone is going to have
to present a mechanism for how a VM could even figure out that it read
junk from the regsiters...

>
> --- a/xen/include/xen/lib/x86/cpu-policy.h
> +++ b/xen/include/xen/lib/x86/cpu-policy.h
> @@ -121,7 +121,31 @@ struct cpu_policy
>              uint64_t :64, :64; /* Leaf 0x3 - PSN. */
>              uint64_t :64, :64; /* Leaf 0x4 - Structured Cache. */
>              uint64_t :64, :64; /* Leaf 0x5 - MONITOR. */
> -            uint64_t :64, :64; /* Leaf 0x6 - Therm/Perf. */
> +
> +            /* Leaf 0x6 - Therm/Perf. */
> +            struct {
> +                uint32_t /* a */:1,
> +                    turbo:1,
> +                    arat:1,
> +                    :4,
> +                    hwp:1,
> +                    hwp_notification:1,
> +                    hwp_activity_window:1,
> +                    hwp_epp:1,
> +                    hwp_plr:1,
> +                    :1,
> +                    hdc:1,
> +                    :2,
> +                    hwp_peci:1,
> +                    :2,
> +                    hw_feedback:1,
> +                    :12;
> +                uint32_t /* b */:32;
> +                uint32_t /* c */ aperfmperf:1,
> +                    :31;
> +                uint32_t /* d */:32;

Elsewhere, single bit fields are bool foo:1, and these want to match for
consistency.  In particular using uint32_t:1 creates a latent bug in
patch 8.

One problem with bool bitfields is that your :4 needs to become 4x :1. 
Right now his hidden in the macros that gen-cpuid.py makes.

Given that b is of type uint32_t, you can omit the :12 from the end of a
and leave a comment.  Similarly, the trailing :31 on c can be dropped.

> +            } pm;

Nothing else is sub-scoped.  I'd prefer that you drop the 'pm'.

~Andrew