Re: [PATCH v3 1/6] x86/cpu-policy: define bits of leaf 6

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.01.2026 17:55, Teddy Astie wrote:
> Le 14/01/2026 à 14:45, Jan Beulich a écrit :
>> ... 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).
>> ---
>> v3: Use SDM-conforming names. (Sorry Jason, had to drop you R-b once
>>      again.)
>> v2: Use bool and unions.
>>
>> --- a/xen/include/xen/lib/x86/cpu-policy.h
>> +++ b/xen/include/xen/lib/x86/cpu-policy.h
>> @@ -121,7 +121,46 @@ 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. */
>> +            union {
>> +                uint32_t _6a;
>> +                struct {
>> +                    bool :1,
>> +                        turbo_boost:1,
>> +                        arat:1,
>> +                        :1,
>> +                        :1,
>> +                        :1,
>> +                        :1,
>> +                        hwp:1,
>> +                        hwp_interrupt:1,
>> +                        hwp_activity_window:1,
>> +                        hwp_epp:1,
>> +                        hwp_request_pkg:1,
>> +                        :1,
>> +                        hdc:1,
>> +                        :1,
>> +                        :1,
>> +                        hwp_peci_override:1,
>> +                        :1,
>> +                        :1,
>> +                        hw_feedback:1;
>> +                };
>> +            };
>> +            union {
>> +                uint32_t _6b;
>> +            };
>> +            union {
>> +                uint32_t _6c;
>> +                struct {
>> +                    bool hw_feedback_cap:1;
>> +                };
>> +            };
>> +            union {
>> +                uint32_t _6d;
>> +            };
>> +
> 
> While I'm ok for the a and c unions, I'm not convinced by the b and d 
> ones (union with just a single uint32_t in it) as it's quite verbose and 
> inconsistent with the rest of the cpu_policy structure.

Indeed for them I wasn't quite certain. I could drop the union wrapping
for now (until individual fields appear), yet then I'd again be on the
edge: Use

            uint32_t _6b;

or

            uint32_t :32;

? Both have their pros and cons. Hence why I went with consistent layout
for all 4 fields. If there was a clear majority preference for either of
the above, I'd be fine to switch.

Jan