Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play_dead()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 15, 2024 at 02:21:19AM +0100, Thomas Gleixner wrote:
> On Thu, Nov 14 2024 at 13:03, Peter Zijlstra wrote:
> > 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.
> 
> Instead of playing dead or to kick out CPUs from whatever dead play
> routine they are in?
> 
> playimg dead is to stay because INIT will bring back the MCE broadcast
> problem, which we try to avoid by bringing SMT siblings up just to shut
> them down again by playing dead.
> 
> You need a MCE broadcast free system and/or some sensible BIOS bringup
> code for that to work...

Isn't INIT a better state to be in during kexec than HLT?