linux-kernel - Re: [PATCH] x86, fpu: avoid FPU lazy restore after suspend

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Date:	Fri, 30 Nov 2012 11:38:25 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Vincent Palatin <vpalatin@...omium.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Duncan Laurie <dlaurie@...omium.org>,
	Olof Johansson <olofj@...omium.org>
Subject: Re: [PATCH] x86, fpu: avoid FPU lazy restore after suspend

On 11/30/2012 11:25 AM, Linus Torvalds wrote:
> 
> and in fact I think the right place to do this *might* be in
> "native_cpu_die()" instead, at which point it would actually be
> something like
> 
>     per_cpu(fpu_owner_task, cpu) = NULL;
> 
> *after* the CPU is dead, so that nothing ever can actually see the
> state where a process is still running on the CPU and might possibly
> use the FPU.
> 
> I dunno. I think doing it after really killing the CPU (ie in the
> native_cpu_die() function) might be easier to think about, but I don't
> really hate your patch either (it does make me go "ok, we need to
> guarantee no scheduling or FP use after" - which is probably true, but
> it's still some non-local thing). Either way, a comment about it and
> abstracting whatever the invalidation sequence is in fpu-internal.h
> sounds like a good idea.
> 

Hmm... from my point of view it would almost seem saner to do this on
the way *up*... as part of CPU (re-)initialization.  After all, the
"nothing is currently running on this CPU" is part of the initial state
of the CPU, regardless of if we have ever been online before or not.

	-hpa

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