linux-kernel - Re: [PATCH 4/8] unify paravirt parts of system.h

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Message-Id: <200712170127.31615.rjw@sisk.pl>
Date:	Mon, 17 Dec 2007 01:27:29 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pavel Machek <pavel@....cz>, Andi Kleen <ak@...e.de>,
	Avi Kivity <avi@...ranet.com>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	glommer@...il.com, tglx@...utronix.de, ehabkost@...hat.com,
	jeremy@...p.org, anthony@...emonkey.ws,
	virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	chrisw@...s-sol.org, rostedt@...dmis.org, hpa@...or.com
Subject: Re: [PATCH 4/8] unify paravirt parts of system.h

On Saturday, 15 of December 2007, Ingo Molnar wrote:
> 
> * Pavel Machek <pavel@....cz> wrote:
> 
> > > Linux never uses that register. The only user is suspend 
> > > save/restore, but that' bogus because it wasn't ever initialized by 
> > > Linux in the first place. It could be probably all safely removed.
> > 
> > It probably is safe to remove... but we currently support '2.8.95 
> > kernel loads/resumes 2.6.24 image'... which would break if 2.8 uses 
> > cr8.
> > 
> > So please keep it if it is not a big problem.
> 
> hm, so __save_processor_state() is in essence an ABI? Could you please 
> also send a patch that documents this prominently, in the structure 
> itself?

Hmm, I'm not sure if it really is an ABI part.  It doesn't communicate anything
outside of the kernel in which it is defined.

The problem is, though, that if kernel A is used for resuming kernel B, and
kernel B doesn't save/restore everything it will need after the resume, then
things will break if kernel A modifies that.  So, yes, we'll need to document
that explicitly.

Greetings,
Rafael
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