linux-kernel - Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play

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Message-ID: <20241114120315.GG38972@noisy.programming.kicks-ass.net>
Date: Thu, 14 Nov 2024 13:03:15 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	rafael.j.wysocki@...el.com, len.brown@...el.com,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer
 cpuidle_play_dead() over mwait_play_dead()

On Tue, Nov 12, 2024 at 03:01:27PM +0100, Peter Zijlstra wrote:

> No, not mwait hint. We need an instruction that:
> 
>  - goes to deepest C state
>  - drops into WAIT-for-Start-IPI (SIPI)
> 
> Notably, it should not wake from:
> 
>  - random memory writes
>  - NMI, MCE, SMI and other such non-maskable thingies
>  - anything else -- the memory pointed to by RIP might no longer exist
> 
> Lets call the instruction: DEAD.

So, turns out that when you send INIT to an AP it does the whole drop
into Wait-for-SIPI and ignore non-maskable crap.

The reason we don't do that is because INIT to CPU0 (BP) is somewhat
fatal, but since Thomas killed all that CPU0 hotplug crap, I think we
can actually go do that.