Re: [PATCH v3 1/3] x86: Allow limiting the max C-state sub-state

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 07/23/2014 08:34 AM, Jan Beulich wrote:
>>>> On 07.07.14 at 17:14, <ross.lagerwall@citrix.com> wrote:
>> max_csubstate is implemented in the same way as max_cstate.  However,
>> given the states in the Bay Trail patch (specifically C6N-BYT and
>> C6S-BYT), this is clearly insufficient.
>> How do you think max_csubstate should limit the substate? Should it be
>> something like a max_csubstate = 2 permits the first and second substates?
>
> It's not at all clear to me how to properly translate the CPU internal
> state identifiers to/from command line option values, in a forward
> compatible manner.
>

Well, given that Xen treats the cpuidle_state array as a linear sequence 
of states, and they are ordered by exit latency/target residency, one 
approach is to have the command-line parameters control how deep down 
into the state array the processor is allowed to travel, as was my first 
approach. I know you rejected this before because it was 
hardware-specific and broke the notion of a C-state but I still feel 
it's the simplest solution given that any other way of mapping to CPU 
internal states is likely to be more complex and even more hardware 
dependent...

Regards
-- 
Ross Lagerwall