Re: [PATCH v3 02/15] xen/x86: introduce new sub-hypercall to propagate CPPC data

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2025 09:27, Penny, Zheng wrote:
> [Public]
> 
> Hi,
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: Tuesday, March 25, 2025 3:54 PM
>> To: Penny, Zheng <penny.zheng@xxxxxxx>
>> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; Andrew Cooper
>> <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>;
>> Anthony PERARD <anthony.perard@xxxxxxxxxx>; Orzel, Michal
>> <Michal.Orzel@xxxxxxx>; Julien Grall <julien@xxxxxxx>; Stefano Stabellini
>> <sstabellini@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH v3 02/15] xen/x86: introduce new sub-hypercall to 
>> propagate
>> CPPC data
>>
>> On 25.03.2025 05:12, Penny, Zheng wrote:
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Sent: Monday, March 24, 2025 10:28 PM
>>>>
>>>> On 06.03.2025 09:39, Penny Zheng wrote:
>>>>> +    pm_info = processor_pminfo[cpuid];
>>>>> +    /* Must already allocated in set_psd_pminfo */
>>>>> +    if ( !pm_info )
>>>>> +    {
>>>>> +        ret = -EINVAL;
>>>>> +        goto out;
>>>>> +    }
>>>>> +    pm_info->cppc_data = *cppc_data;
>>>>> +
>>>>> +    if ( cpufreq_verbose )
>>>>> +        print_CPPC(&pm_info->cppc_data);
>>>>> +
>>>>> +    pm_info->init = XEN_CPPC_INIT;
>>>>
>>>> That is - whichever Dom0 invoked last will have data recorded, and
>>>> the other effectively is discarded? I think a warning (perhaps a
>>>> one-time one) is minimally needed to diagnose the case where one type of
>> data replaces the other.
>>>>
>>>
>>> In last v2 discussion, we are discussing that either set_px_pminfo or
>>> set_cppc_pminfo shall be invoked, which means either PX data is recorded, or
>> CPPC data is recorded.
>>> Current logic is that, cpufreq cmdline logic will set the
>>> XEN_PROCESSOR_PM_PX/CPPC flag to reflect user preference, if user
>>> defines the fallback option, like "cpufreq=amd-cppc,xen", we will have both
>> XEN_PROCESSOR_PM_PX | XEN_PROCESSOR_PM_CPPC set in the
>> beginning.
>>> Later in cpufreq driver register logic, as only one register could be
>>> registered , if amd-cppc being registered successfully, it will clear the
>> XEN_PROCESSOR_PM_PX flag bit.
>>> But if it fails to register, fallback scheme kicks off, we will try
>>> the legacy P-states, in the mean time, clearing the
>> XEN_PROCESSOR_PM_CPPC.
>>> We are trying to make XEN_PROCESSOR_PM_PX and
>> XEN_PROCESSOR_PM_CPPC
>>> exclusive values after driver registration, which will ensure us that
>>> either set_px_pminfo or set_cppc_pminfo is taken in the runtime.
>>
>> Yet you realize that this implies Dom0 to know what configuration Xen uses, 
>> in
>> order to know which data to upload. The best approach might be to have
>> Dom0 upload all data it has, with us merely ignoring what we can't make use 
>> of.
> 
> PLZ correct me if I understand you wrongly:
> Right now, I was letting DOM0 upload all data it has, and in the Xen:
> ```
>     case XEN_PM_CPPC:
>         if ( !(xen_processor_pmbits & XEN_PROCESSOR_PM_CPPC) )
>         {
>             ret = -EOPNOTSUPPED;
>             break;
>         }
>         ret = set_cppc_pminfo(op->u.set_pminfo.id,
>                               &op->u.set_pminfo.u.cppc_data);
>         break;
> 
>     case XEN_PM_PX:
>         if ( !(xen_processor_pmbits & XEN_PROCESSOR_PM_PX) )
>         {
>             ret = -EOPNOTSUPPED;
>             break;
>         }
>         ret = set_px_pminfo(op->u.set_pminfo.id, &op->u.set_pminfo.u.perf);
>         break;
> ```
> I relied on flag XEN_PROCESSOR_PM_CPPC and XEN_PROCESSOR_PM_PX to choose which
> info we shall record.
> Firstly, we shall not return -EOPNOTSUPPED error above there.

Yes.

>> The order of uploading (CPPC first or CPPC last) shouldn't matter. Then (and 
>> only
>> then, and - ftaod - only when uploading of the "wrong" kind of data doesn't 
>> result in
>> an error) things can go without warning.
> 
> Then in
> ```
>     pm_info->init = XEN_CPPC_INIT;
>     ret = cpufreq_cpu_init(cpuid);
> ```
> We shall add warning here to clarify no fallback scheme to replace now, when 
> ret is not zero.

Maybe. In the earlier reply I said with certain conditions fulfilled a warning
may not be necessary. Yet perhaps initially having a warning there (maybe just
for debug builds) may make sense.

Jan