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: <0ecea0e5be59e63b7827f4db368f2aa3322fb71d.camel@linux.intel.com>
Date: Tue, 12 Nov 2024 14:44:49 +0200
From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki"
	 <rafael@...nel.org>
Cc: 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, 2024-11-12 at 13:18 +0100, Peter Zijlstra wrote:
> But on Intel we really don't want HLT, and had that MWAIT, but that has
> real problems with KEXEC. And I don't think we can rely on INTEL_IDLE=y.

If INTEL_IDLE is not set, then we'll just use existing mwait creation algorithm
in 'mwait_play_dead()', which works too, just not ideal.


> Anyway, ideally x86 would grow a new instruction to offline a CPU, both
> MWAIT and HLT have problems vs non-maskable interrupts.
... snip ...
> But as said, we need a new instruction.

FYI, I already started discussing a special "gimme the deepest C-state" mwait
hint - just a constant like 0xFF. CPUID leaf 5 has many reserved bits, one could
be used for enumeration of this feature.

But this is just a quick idea so far, and informal discussions so far.

Artem.