Re: [PATCH 3/4] mwait-idle: add 'preferred_cstates' module argument

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.04.2022 17:06, Roger Pau Monné wrote:
> On Wed, Apr 27, 2022 at 03:41:24PM +0200, Jan Beulich wrote:
>> On 27.04.2022 14:45, Roger Pau Monné wrote:
>>> On Tue, Apr 26, 2022 at 12:05:28PM +0200, Jan Beulich wrote:
>>>> --- unstable.orig/xen/arch/x86/cpu/mwait-idle.c
>>>> +++ unstable/xen/arch/x86/cpu/mwait-idle.c
>>>> @@ -82,6 +82,18 @@ boolean_param("mwait-idle", opt_mwait_id
>>>>  
>>>>  static unsigned int mwait_substates;
>>>>  
>>>> +/*
>>>> + * Some platforms come with mutually exclusive C-states, so that if one is
>>>> + * enabled, the other C-states must not be used. Example: C1 and C1E on
>>>> + * Sapphire Rapids platform. This parameter allows for selecting the
>>>> + * preferred C-states among the groups of mutually exclusive C-states - 
>>>> the
>>>> + * selected C-states will be registered, the other C-states from the 
>>>> mutually
>>>> + * exclusive group won't be registered. If the platform has no mutually
>>>> + * exclusive C-states, this parameter has no effect.
>>>> + */
>>>> +static unsigned int __ro_after_init preferred_states_mask;
>>>> +integer_param("preferred-cstates", preferred_states_mask);
>>>> +
>>>>  #define LAPIC_TIMER_ALWAYS_RELIABLE 0xFFFFFFFF
>>>>  /* Reliable LAPIC Timer States, bit 1 for C1 etc. Default to only C1. */
>>>>  static unsigned int lapic_timer_reliable_states = (1 << 1);
>>>> @@ -96,6 +108,7 @@ struct idle_cpu {
>>>>    unsigned long auto_demotion_disable_flags;
>>>>    bool byt_auto_demotion_disable_flag;
>>>>    bool disable_promotion_to_c1e;
>>>> +  bool enable_promotion_to_c1e;
>>>
>>> I'm confused by those fields, shouldn't we just have:
>>> promotion_to_c1e = true | false?
>>>
>>> As one field is the negation of the other:
>>> enable_promotion_to_c1e = !disable_promotion_to_c1e
>>>
>>> I know this is code from Linux, but would like to understand why two
>>> fields are needed.
>>
>> This really is a tristate; Linux is now changing their global variable
>> to an enum, but we don't have an equivalent of that global variable.
> 
> So it would be: leave default, disable C1E promotion, enable C1E
> promotion.
> 
> And Linux is leaving the {disable,enable}_promotion_to_c1e in
> idle_cpu?

Iirc they only have disable_promotion_to_c1e there (as a struct field)
and keep it, but they convert the similarly named file-scope variable
to a tristate.

> I guess there's not much we can do unless we want to diverge from
> upstream.

We've diverged some from Linux here already - as said, for example we
don't have their file-scope variable. I could convert our struct field
to an enum, but that would be larger code churn for (I think) little
gain.

Jan