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

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Message-ID: <20071205163017.GA4756@ucw.cz>
Date:	Wed, 5 Dec 2007 16:30:17 +0000
From:	Pavel Machek <pavel@....cz>
To:	Andi Kleen <ak@...e.de>
Cc:	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, mingo@...e.hu,
	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,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH 4/8] unify paravirt parts of system.h

On Tue 2007-12-04 20:34:32, Andi Kleen wrote:
> On Tue, Dec 04, 2007 at 09:18:33PM +0200, Avi Kivity wrote:
> > Glauber de Oliveira Costa wrote:
> >> This patch moves the i386 control registers manipulation functions,
> >> wbinvd, and clts functions to system.h. They are essentially the same
> >> as in x86_64, except for the cr8 register, which we add.
> >>
> >> +
> >> +static inline unsigned long native_read_cr8(void)
> >> +{
> >> +	unsigned long cr8;
> >> +	asm volatile("mov %%cr8,%0" : "=r" (cr8), "=m" (__force_order));
> >> +	return cr8;
> >> +}
> >> +
> >>   
> >
> > There is no cr8 register on i386.  This had better be protected by an 
> > #ifdef.
> >
> > (you're likely not getting an error since it's a static inline, so the asm 
> > is never emitted)
> 
> 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.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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