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

[Jan Beulich <jbeulich@xxxxxxxx>]

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. 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.

Jan