Re: [PATCH v2 05/11] xen/x86: introduce a new amd cppc driver for cpufreq scaling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.02.2025 07:53, Penny, Zheng wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: Wednesday, February 12, 2025 12:46 AM
>>
>> On 06.02.2025 09:32, Penny Zheng wrote:
>>> --- a/xen/arch/x86/acpi/cpufreq/amd-cppc.c
>>> +++ b/xen/arch/x86/acpi/cpufreq/amd-cppc.c
>>> +}
>>> +
>>> +static int cf_check amd_cppc_write_request(int cpu, uint8_t min_perf,
>>> +                                           uint8_t des_perf, uint8_t
>>> +max_perf) {
>>> +    struct amd_cppc_drv_data *data = per_cpu(amd_cppc_drv_data, cpu);
>>> +    uint64_t prev = data->req.raw;
>>> +
>>> +    data->req.min_perf = min_perf;
>>> +    data->req.max_perf = max_perf;
>>> +    data->req.des_perf = des_perf;
>>> +
>>> +    if ( prev == data->req.raw )
>>> +        return 0;
>>> +
>>> +    on_selected_cpus(cpumask_of(cpu), amd_cppc_write_request_msrs,
>>> + data, 1);
>>> +
>>> +    return data->err;
>>> +}
>>
>> ... in this function. Then the field would be written to (and the cacheline 
>> change
>> ownership) only in the error case.
>>
>> As to the function's parameters - why's there a plain int?
> 
> Are you asking why "int cpu" here?

Yes. I don't expect negative values are okay to be passed in? And variables
(incl parameters) which can't hold negative values want to be of an unsigned
type.

>>> + err:
>>> +    data->err = -EINVAL;
>>> +}
>>
>> At this point you may have set the enable bit already, which you can't undo.
>> What effect will this have on the system when the error path is taken here?
>> Especially if we then try to fall back to another driver?
> 
> On current code logic, when the error path is taken, amd-cppc cpufreq driver 
> fails
> to initialize. And we will also not fall back to another driver.
> As we could register *only one* cpufreq driver. If amd-cppc is registered 
> properly
> before, then legacy P-states will not get registered.
> In amd-cppc registration code:
> ```
> +int __init amd_cppc_register_driver(void)
> +{
> +    if ( !cpu_has_cppc )
> +        return -ENODEV;
> +
> +    return cpufreq_register_driver(&amd_cppc_cpufreq_driver);
> +}
> ```
> CPPC feature MSR gets checked before the registration, which is to check 
> whether the
> hardware has CPPC msr support.

Yet still there's the possibility of a later error. If it's not an option to
gracefully handle such errors, I think you want to put in a code comment
explaining the situation and effect.

Jan