Re: [PATCH v4 05/15] xen/x86: introduce "cpufreq=amd-cppc" xen cmdline

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.05.2025 10:28, Penny, Zheng wrote:
> [Public]
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: Monday, May 19, 2025 9:19 PM
>> To: Penny, Zheng <penny.zheng@xxxxxxx>
>> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; Andrew Cooper
>> <andrew.cooper3@xxxxxxxxxx>; Anthony PERARD <anthony.perard@xxxxxxxxxx>;
>> Orzel, Michal <Michal.Orzel@xxxxxxx>; Julien Grall <julien@xxxxxxx>; Roger 
>> Pau
>> Monné <roger.pau@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; 
>> xen-
>> devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH v4 05/15] xen/x86: introduce "cpufreq=amd-cppc" xen
>> cmdline
>>
>> On 19.05.2025 10:36, Penny, Zheng wrote:
>>> [Public]
>>>
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Sent: Tuesday, April 29, 2025 8:52 PM
>>>> To: Penny, Zheng <penny.zheng@xxxxxxx>
>>>> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; Andrew Cooper
>>>> <andrew.cooper3@xxxxxxxxxx>; Anthony PERARD
>>>> <anthony.perard@xxxxxxxxxx>; Orzel, Michal <Michal.Orzel@xxxxxxx>;
>>>> Julien Grall <julien@xxxxxxx>; Roger Pau Monné
>>>> <roger.pau@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
>>>> xen- devel@xxxxxxxxxxxxxxxxxxxx
>>>> Subject: Re: [PATCH v4 05/15] xen/x86: introduce "cpufreq=amd-cppc"
>>>> xen cmdline
>>>>
>>>> On 14.04.2025 09:40, Penny Zheng wrote:
>>>>> --- a/xen/include/acpi/cpufreq/processor_perf.h
>>>>> +++ b/xen/include/acpi/cpufreq/processor_perf.h
>>>>> @@ -5,6 +5,9 @@
>>>>>  #include <public/sysctl.h>
>>>>>  #include <xen/acpi.h>
>>>>>
>>>>> +/* ability bits */
>>>>> +#define XEN_PROCESSOR_PM_CPPC   8
>>>>
>>>> This needs correlating (at least via commentary, better by build-time
>>>> checking) with the other XEN_PROCESSOR_PM_* values. Otherwise
>> someone
>>>> adding a new #define in the public header may not (easily) notice a
>>>> possible conflict. With that in mind I also question whether 8 is
>>>> actually a good choice: That's the obvious next value to use in the
>>>> public interface. SIF_PM_MASK is 8 bits wide, so a sensible value to use 
>>>> here
>> would by e.g. 0x100.
>>>>
>>>
>>> I've added a public flag anchor "XEN_PROCESSOR_PM_PUBLIC_END" in
>> public header:
>>>          #define XEN_PROCESSOR_PM_PUBLIC_END
>> XEN_PROCESSOR_PM_TX
>>> and will do the following build-time checking:
>>>         BUILD_BUG_ON(XEN_PROCESSOR_PM_CPPC <=
>>> XEN_PROCESSOR_PM_PUBLIC_END);
>>
>> I don't really see why anything would need to be added to the public header 
>> (and we
>> really should strive to avoid unnecessary additions).
> 
> If I put such definition
> "#define XEN_PROCESSOR_PM_PUBLIC_END XEN_PROCESSOR_PM_TX"
> in internal header, I'm afraid it won't be updated if new ones added in the 
> public in the future.
> Or any other suggestions to provide build-time checking?

Imo it's sufficient to check that the new #define doesn't overlap with
SIF_PM_MASK.

Jan