linux-kernel - Re: [PATCH] x86: Construct 32 bit boot time page tables in native format.

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Message-ID: <m163xii2nm.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 25 Jan 2008 04:07:57 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Ian Campbell <ijc@...lion.org.uk>, Ingo Molnar <mingo@...e.hu>,
	Mika Penttilä <mika.penttila@...umbus.fi>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] x86: Construct 32 bit boot time page tables in native format.

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Eric W. Biederman wrote:
>> Note.  I don't believe we use either trampoline (cpu startup or acpi wakeup)
>> in the hypervisor case (esp Xen).  So we should be able to completely ignore
>> Xen and do the memcpy of pgd entries.
>>
>
> Indeed.  The alias mapping can be set up in native_pagetable_setup_done() and
> needn't involve Xen at all.

Good.  Then this case gets easy.

We just need a pgd that has pgd entries that duplicate the kernel pgd entries
at both address 0 and at the normal kernel address.

In 64bit mode we make this part of the trampoline because we need a pgt below
4G so that we can point a 32bit %cr3 value at it.  We can either use that
technique for the 32bit kernel (and be consistent) or we can have a single
trampoline/wakeup pgd that we use.  As all pgd entries must be below 4G in
32bit mode.

Although if we really wanted to be restrictive we could have a much more limited
set of identity page table entries that only map the low 1M, or possibly just
640K.

Eric
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