[Xen-devel] Re: [PATCH 1/3] x86: calculate precisely the memory needed by init_memory_mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 20 Jun 2011, H. Peter Anvin wrote:
> On 06/07/2011 11:13 AM, stefano.stabellini@xxxxxxxxxxxxx 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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