linux-kernel - Re: [PATCH 04/40] xen: document Xen is using an unused bit for the pagetables

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Date:	Thu, 5 Jul 2012 01:05:47 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Alex Shi <alex.shi@...el.com>,
	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Don Morris <don.morris@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 04/40] xen: document Xen is using an unused bit for the
 pagetables

On Fri, Jun 29, 2012 at 10:16:12AM -0400, Rik van Riel wrote:
> On 06/28/2012 08:55 AM, Andrea Arcangeli wrote:
> > Xen has taken over the last reserved bit available for the pagetables
> > which is set through ioremap, this documents it and makes the code
> > more readable.
> >
> > Signed-off-by: Andrea Arcangeli<aarcange@...hat.com>
> > ---
> >   arch/x86/include/asm/pgtable_types.h |   11 +++++++++--
> >   1 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > index 013286a..b74cac9 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -17,7 +17,7 @@
> >   #define _PAGE_BIT_PAT		7	/* on 4KB pages */
> >   #define _PAGE_BIT_GLOBAL	8	/* Global TLB entry PPro+ */
> >   #define _PAGE_BIT_UNUSED1	9	/* available for programmer */
> > -#define _PAGE_BIT_IOMAP		10	/* flag used to indicate IO mapping */
> > +#define _PAGE_BIT_UNUSED2	10
> 
> Considering that Xen is using it, it is not really
> unused, is it?

_PAGE_BIT_UNUSED1 is used too (_PAGE_BIT_SPECIAL). Unused stands for
unused by the CPU, not by the OS. But this patch is dropped.
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