linux-kernel - Re: [PATCH v3 02/11] x86/kexec: Add extra pointers to transition page table PGD, PUD, PMD and PTE

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Date:	Wed, 9 Jan 2013 19:42:53 +0100
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	andrew.cooper3@...rix.com, x86@...nel.org, tglx@...utronix.de,
	kexec@...ts.infradead.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel <xen-devel@...ts.xen.org>, konrad.wilk@...cle.com,
	maxim.uvarov@...cle.com, mingo@...hat.com, vgoyal@...hat.com,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com, hpa@...or.com
Subject: Re: [PATCH v3 02/11] x86/kexec: Add extra pointers to transition
 page table PGD, PUD, PMD and PTE

On Mon, Jan 07, 2013 at 01:05:10PM +0000, Jan Beulich wrote:
> >>> On 07.01.13 at 13:52, Daniel Kiper <daniel.kiper@...cle.com> wrote:
> > On Mon, Jan 07, 2013 at 09:48:20AM +0000, Jan Beulich wrote:
> >> >>> On 04.01.13 at 18:25, Daniel Kiper <daniel.kiper@...cle.com> wrote:
> >> > Right, so where is virtual mapping of control page established?
> >> > I could not find relevant code in SLES kernel which does that.
> >>
> >> In the hypervisor (xen/arch/x86/machine_kexec.c:machine_kexec_load()).
> >> xen/arch/x86/machine_kexec.c:machine_kexec() then simply uses
> >> image->page_list[1].
> >
> > This (xen/arch/x86/machine_kexec.c:machine_kexec_load()) maps relevant
> > page (allocated earlier by dom0) in hypervisor fixmap area. However,
> > it does not make relevant mapping in transition page table which
> > leads to crash when %cr3 is switched from Xen page table to
> > transition page table.
>
> That indeed could explain _random_ failures - the fixmap entries
> get created with _PAGE_GLOBAL set, i.e. don't get flushed with
> the CR3 write unless CR4.PGE is clear.

This does not matter. As I stated earlier virtual mapping is wrong.
relocate_kernel() is mapped at its virtual address in Linux kernel
instead of control page at its virtual address in Xen hypervisor.
I tested SLES kernel once again. It does not work.

> And I don't see how your allocation of intermediate page tables
> would help: You wouldn't know where the mapping of the control
> page lives until you're actually in the early relocate_kernel code.

Right. Allocation itself is not a solution for this problem.
It should be acompanied by code which establishes transition
page table in relocate_kernel() (which is later copied
to control page, i.e. code of relocate_kernel()).

> Or was it that what distinguishes your cloned code from the
> native original?

No, my code is based on native original.
There are some implementation differences.

Daniel
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