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Date:      Fri, 02 Aug 2002 20:12:37 -0700
From:      Peter Wemm <peter@wemm.org>
To:        David Greenman-Lawrence <dg@dglawrence.com>
Cc:        cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org
Subject:   Re: cvs commit: src/sys/i386/i386 pmap.c 
Message-ID:  <20020803031237.78CDA2A7D6@canning.wemm.org>
In-Reply-To: <20020802181752.C54567@nexus.root.com> 

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David Greenman-Lawrence wrote:
> >peter       2002/08/02 18:02:37 PDT
> >
> >  Modified files:
> >    sys/i386/i386        pmap.c 
> >  Log:
> >  Take advantage of the fact that there is a small 1MB direct mapped region
> >  on x86 in between KERNBASE and the kernel load address.  pmap_mapdev()
> >  can return pointers to this for devices operating in the isa "hole".
> 
>    Actually the mapping of the low 1MB of physical address space was a Jolitz
> abortion which I had hoped to someday remove.
> 
> -DG

Indeed.  I do not particularly like it but it seemed fair game for this
well contained case.  It isn't exposed outside of pmap.c and is dead simple
to turn back off again.  Mapping this 1MB block of memory at KERNBASE and
then also adding it to the general page pool does make me a bit nervous.

On a similar topic, we have a couple of weird TLB things happening
on certain ppro and early pentiumII steppings that look suspiciously
related to one of the errata that boils down to 'Do Not point a 4MB page at
physical address zero.'  While we are not doing exactly what is described
there, I have noticed that the wierd(TM) things happen to happen on the
same steppings so far.  Admittedly it was from a very small sample. I'd
like to be able to configure the kernel to load at 4MB instead of 1MB and
use whole 4MB pages for the kernel and the initial data, instead of
effectively losing 1MB from the 4MB tlb slot.  That requires a cleanup of a
fair few places, including some drivers that "know" the relationship between
the isa hole and KERNBASE.  Even vm86 "knows" more than is good for it.

Cheers,
-Peter
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5


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