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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 23 Jul 2016 10:16:45 +0900
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Subject: Re: [PATCH] x86/boot: Reorganize and clean up the BIOS area
 reservation code
To: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Andy Lutomirski <luto@amacapital.net>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
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        Kees Cook <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, X86 ML <x86@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>, Peter Jones <pjones@redhat.com>
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On Fri, Jul 22, 2016 at 10:00 PM, Matt Fleming <matt@codeblueprint.co.uk> wrote:
>
> I suppose we could rewrite the page table mapping for those precious
> <1MB regions to coerce the firmware into accessing different pages
> instead of the 1:1 addresses and copy the regions elsewhere. Maybe.
> That assumes we don't hit other firmware bugs though.

.. it also assumes that firmware honors our page tables. Which at a
minimum won't be true for things like SMM, which happens with paging
entirely disabled.

And that's exactly the kind of code that touches some of the memory in
the low 1M region - things like the legacy keyboard state bits in the
low 4kB etc, for crazy old DOS days, iirc.

                Linus