linux-kernel - Re: [PATCH] x86_64: disable identity mappings statically

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Date:	Fri, 11 Feb 2011 18:07:22 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64: disable identity mappings statically

On Fri, Feb 11, 2011 at 08:46:41AM -0800, Eric W. Biederman wrote:
> Jiri Olsa <jolsa@...hat.com> writes:
> 
> > hi,
> >
> > while browsing the page table setup code, I noticed the x86_64 head
> > code might not need the identity mappings at all.
> > It seems it's ok to switch it off completely from the begining,
> > unless I'm missing something.
> 
> Have you tested it?

yes, I booted it with no problem

> 
> I expect you will find that we need the identity mapping because
> before we load this page table we are running with virt==phys
> and we need the identity mapping retained  in the new page table
> so we can get to the instruction after movq %rax, %cr0.

well, right after the page table setup, there's following code
switching to the kernel map adresses 0xffffffff80000000+

        movq    %rax, %cr3

        /* Ensure I am executing from virtual addresses */
        movq    $1f, %rax
        jmp     *%rax
1:


and I found no other identity mapping usage after this point

jirka
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