linux-kernel - Re: [Patch V0] x86, mce: Ensure offline CPU's don't participate in mce rendezvous process.

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Date:	Fri, 4 Dec 2015 19:08:21 -0500
From:	"Raj, Ashok" <ashok.raj@...el.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Luck, Tony" <tony.luck@...el.com>, Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>
Subject: Re: [Patch V0] x86, mce: Ensure offline CPU's don't participate in
 mce rendezvous process.

On Fri, Dec 04, 2015 at 02:34:52PM -0800, Andy Lutomirski wrote:
> On Fri, Dec 4, 2015 at 9:53 AM, Luck, Tony <tony.luck@...el.com> wrote:
> > ist_enter() is black magic to me. Andy? Would you be worried about executing
> > ist_{enter,exit}() on a cpu that was once online, but is currently marked offline
> > by Linux?
> 
> Offline CPUs are black magic to me.  But as long as the CPU works the
> way that the normal specs say it should, then ist_enter is fair game.
> In any event, if context tracking blows up on an offline CPU, I'd
> argue that's a context tracking bug and needs to be fixed.
> 
> But maybe offlined CPUs are supposed to have all interrupts off
> (including MCE?) and the argument goes the other way?  Dunno.

MCE's are broadcast by the hardware and cannot be blocked. Offline
is only a Linux specific state. Now if the offline was a result of an ACPI
event (eject) that triggered the CPU removal (offline in Linux, as it would 
have in a platform that supports true hotplug) then the platform would 
remove this cpu from the broadcast list. 

if kernel were to set CR4.MCE=0 that would cause system shutdown when 
an MCE is broadcast and hits this cpu.

Cheers,
Ashok
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