[SeaBIOS] SeaBIOS PAE experiment
Kevin O'Connor
kevin at koconnor.net
Wed Sep 30 14:03:43 CET 2015
On Tue, Sep 29, 2015 at 10:19:19PM +0200, Rudolf Marek wrote:
> Hi Kevin,
>
> > * The reason why PAE is interesting (instead of standard i386 paging)
> > is that it allows for 64bit mappings and because one can set it up
> > with just a single level page directory of 2MB pages. The single
> > level page directory makes maintaining it much easier.
>
> There is a performance penalty if you do it for first 2MB of phys memory on some
> P6/XeonP4 CPUs. Others could trigger undefined behaviour.
>
> The reason for this are the special regions in the 640KB-1MB covered by various
> MTRRs. See 11.11.9 Large Page Size Considerations in the Intel manual for the
> details.
Thanks - good catch. It looks like that would require using 4KB pages
for identity mapping the first 2MB or ram.
-Kevin
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