linux-kernel - Re: [PATCH 1/3] x86: calculate precisely the memory needed by init_memory

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Message-ID: <alpine.DEB.2.00.1106211601570.12963@kaball-desktop>
Date:	Tue, 21 Jun 2011 18:57:01 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>
Subject: Re: [PATCH 1/3] x86: calculate precisely the memory needed by
 init_memory_mapping

On Mon, 20 Jun 2011, H. Peter Anvin wrote:
> On 06/07/2011 11:13 AM, stefano.stabellini@...citrix.com wrote:
> > 
> > - remove the extra page added on x86_32 for the fixmap because is not
> > needed: the PMD entry is already allocated and contiguous for the whole
> > range (a PMD page covers 4G of virtual addresses) and the pte entry is
> > already allocated by early_ioremap_init.
> > 
> 
> Hi Stefano,
> 
> I think this is wrong.  A PMD page covers *1G* of virtual addresses, and
> in the 2+2 and 1+3 memory configurations, we may or may not need a
> separate PMD for the fixmap.
> 
> Am I missing something?

You are right, a PMD page covers 1G of virtual addresses so that part of
the explanation in the comment is wrong.

The reason why we don't need a separate PMD for the fixmap is that in
both PAE and non-PAE cases the last gigabyte of virtual addresses is
always covered by the initial allocation in head_32.S (swapper_pg_dir or
initial_pg_pmd).
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