linux-kernel - Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI calls

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Date:	Fri, 07 Sep 2012 09:02:11 +0100
From:	Matt Fleming <matt.fleming@...el.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Vasco Dias <rafa.vasco@...il.com>,
	Matthew Garrett <mjg@...hat.com>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, cJ-ko@...gloub.eu
Subject: Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI calls

On Fri, 2012-09-07 at 08:40 +0100, Jan Beulich wrote:
> >>> On 06.09.12 at 19:07, "H. Peter Anvin" <hpa@...or.com> wrote:
> > Anyone care to refresh my memory why we can't use the "global" 
> > 1:1+kernel map, possibly with some improvements (which would benefit the 
> > users of those maps too)?  With that I mean initial_page_map on i386 and 
> > trampoline_pgd on x86-64...
> 
> I don't think you want anything inserted in the low part of those
> (kernel-only) page tables, not even transiently.

There are already things inserted into the low part of those page
tables. In fact, the main purpose is to provide a 1:1 mapping for
realmode code. See machine_real_restart().

I think there's definitely merit to the approach Peter outlined above.
There's a variety of places in arch/x86 where a 1:1 mapping is needed.
It would be good to have a single pagetable that provides this.

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