Re: [PATCH v6 19/19] xen/cpufreq: Adapt SET/GET_CPUFREQ_CPPC xen_sysctl_pm_op for amd-cppc driver

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2025 05:13, Penny, Zheng wrote:
> [Public]
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: Thursday, July 24, 2025 10:44 PM
>> To: Penny, Zheng <penny.zheng@xxxxxxx>
>> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; Anthony PERARD
>> <anthony.perard@xxxxxxxxxx>; Andrew Cooper <andrew.cooper3@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 v6 19/19] xen/cpufreq: Adapt SET/GET_CPUFREQ_CPPC
>> xen_sysctl_pm_op for amd-cppc driver
>>
>> On 11.07.2025 05:51, Penny Zheng wrote:
>>> Introduce helper set_amd_cppc_para() and get_amd_cppc_para() to
>>> SET/GET CPPC-related para for amd-cppc/amd-cppc-epp driver.
>>>
>>> In get_cpufreq_cppc()/set_cpufreq_cppc(), we include
>>> "processor_pminfo[cpuid]->init & XEN_CPPC_INIT" condition check to
>>> deal with cpufreq driver in amd-cppc.
>>>
>>> Also, a new field "policy" has also been added in "struct xen_get_cppc_para"
>>> to describe performance policy in active mode. It gets printed with
>>> other cppc paras. Move manifest constants "XEN_CPUFREQ_POLICY_xxx" to
>>> public header to let it be used in user space tools. Also add a new
>>> anchor "XEN_CPUFREQ_POLICY_xxx" for array overrun check.
>>
>> If only they indeed had XEN_ prefixes.
>>
>>> Signed-off-by: Penny Zheng <Penny.Zheng@xxxxxxx>
>>> ---
>>> v1 -> v2:
>>> - Give the variable des_perf an initializer of 0
>>> - Use the strncmp()s directly in the if()
>>> ---
>>> v3 -> v4
>>> - refactor comments
>>> - remove double blank lines
>>> - replace amd_cppc_in_use flag with XEN_PROCESSOR_PM_CPPC
>>> ---
>>> v4 -> v5:
>>> - add new field "policy" in "struct xen_cppc_para"
>>> - add new performamce policy XEN_CPUFREQ_POLICY_BALANCE
>>> - drop string comparisons with "processor_pminfo[cpuid]->init &
>> XEN_CPPC_INIT"
>>> and "cpufreq.setpolicy == NULL"
>>> - Blank line ahead of the main "return" of a function
>>> - refactor comments, commit message and title
>>> ---
>>> v5 -> v6:
>>> - remove duplicated manifest constants, and just move it to public
>>> header
>>> - use "else if" to avoid confusion that it looks as if both paths
>>> could be taken
>>> - add check for legitimate perf values
>>> - use "unknown" instead of "none"
>>> - introduce "CPUFREQ_POLICY_END" for array overrun check in user space
>>> tools
>>> +         (set_cppc->maximum > data->caps.highest_perf ||
>>> +          set_cppc->maximum < data->caps.lowest_nonlinear_perf) )
>>> +        return -EINVAL;
>>> +    /*
>>> +     * Minimum performance may be set to any performance value in the range
>>> +     * [Nonlinear Lowest Performance, Highest Performance], inclusive but 
>>> must
>>> +     * be set to a value that is less than or equal to Maximum Performance.
>>> +     */
>>> +    if ( set_cppc->set_params & XEN_SYSCTL_CPPC_SET_MINIMUM &&
>>> +         (set_cppc->minimum < data->caps.lowest_nonlinear_perf ||
>>> +          (set_cppc->set_params & XEN_SYSCTL_CPPC_SET_MAXIMUM &&
>>> +           set_cppc->minimum > set_cppc->maximum) ||
>>> +          (!(set_cppc->set_params & XEN_SYSCTL_CPPC_SET_MAXIMUM) &&
>>
>> Hmm, I find this confusing to read, and was first thinking the ! was wrong 
>> here. Imo
>> such is better expressed with the conditional operator:
>>
>>
>>           set_cppc->minimum > (set_cppc->set_params &
>> XEN_SYSCTL_CPPC_SET_MAXIMUM
>>                                ? set_cppc->maximum
>>                                : data->req.max_perf)
>>
> 
> Thx, understood!
> 
>> Which also makes it easier to spot that here you use data->req, when in the
>> minimum check you use data->caps. Why this difference?
>>
> 
>  minimum check has two boundary check,
> left boundary check is against data->caps.lowest_nonlinear_perf. And right 
> boundary check is against data->req.max_perf. As it shall not only not larger 
> than caps.highest_perf , but also req.max_perf. The relation between max_perf 
> and highest_perf is validated in the maximum check. So here, we are only 
> considering max_perf

I still don't get why one check is against capabilities (permitted values) why 
the
other is again what's currently set.

Jan