Re: Ping: [PATCH 0/5] x86: more power-efficient CPU parking

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/08/18 08:08, Jan Beulich wrote:
>>>> On 01.08.18 at 16:22,  wrote:
>> When putting CPUs to sleep permanently, we should try to put them into
>> the most power conserving state possible. For now it is unclear whether,
>> especially in a deep C-state, the P-state also matters, so this series only
>> arranges for the C-state side of things (plus some cleanup).
>>
>> 1: x86/cpuidle: replace a pointless NULL check
>> 2: x86/idle: re-arrange dead-idle handling
>> 3: x86/cpuidle: push parked CPUs into deeper sleep states when possible
>> 4: x86/cpuidle: clean up Cx dumping
>> 5: x86: place non-parked CPUs into wait-for-SIPI state after offlining

I don't have a problem in principle, but I'm afraid that you're going to
need either positive documentation, or a positive statement from Intel
and AMD that wait-for-SIPI is the most power efficient state to park a
CPU in.

At a guess, I'd expect that wait-for-SIPI isn't an optimised state
(because the vendors wouldn't expect hardware to be in this state after
boot), and from the various discussions around L1TF, there are
definitely differences between wait-for-SIPI and mwait.

~Andrew

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