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:	Mon, 07 Jan 2013 13:05:10 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Daniel Kiper" <daniel.kiper@...cle.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 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.

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

Jan

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