linux-kernel - Re: [Xen-devel] crash tool - problem with new Xen linear virtual mapped sparse p2m list

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Date:	Tue, 24 Nov 2015 06:39:17 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Petr Tesarik" <ptesarik@...e.cz>
Cc:	<olaf@...fle.de>, "Andrew Cooper" <andrew.cooper3@...rix.com>,
	"David Vrabel" <david.vrabel@...rix.com>,
	"Malcolm Crossley" <malcolm.crossley@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>,
	"DanielKiper" <daniel.kiper@...cle.com>, <anderson@...hat.com>,
	<crash-utility@...hat.com>, "David Mair" <DMAIR@...e.com>,
	"Juergen Gross" <JGross@...e.com>, <dslutz@...izon.com>,
	<linux-kernel@...r.kernel.org>, <keir@....org>
Subject: Re: [Xen-devel] crash tool - problem with new Xen linear
 virtual mapped sparse p2m list

>>> On 24.11.15 at 13:57, <ptesarik@...e.cz> wrote:
> V Tue, 24 Nov 2015 10:35:03 +0000
> Andrew Cooper <andrew.cooper3@...rix.com> napsáno:
> 
>> On 24/11/15 10:17, Petr Tesarik wrote:
>> > On Tue, 24 Nov 2015 10:09:01 +0000
>> > David Vrabel <david.vrabel@...rix.com> wrote:
>> >
>> >> On 24/11/15 09:55, Malcolm Crossley wrote:
>> >>> On 24/11/15 08:59, Jan Beulich wrote:
>> >>>>>>> On 24.11.15 at 07:55, <JGross@...e.com> wrote:
>> >>>>> What about:
>> >>>>>
>> >>>>> 4) Instead of relying on the kernel maintained p2m list for m2p
>> >>>>>    conversion use the hypervisor maintained m2p list which should be
>> >>>>>    available in the dump as well. This is the way the alive kernel is
>> >>>>>    working, so mimic it during crash dump analysis.
>> >>>> I fully agree; I have to admit that looking at the p2m when doing page
>> >>>> table walks for a PV Dom0 (having all machine addresses in page table
>> >>>> entries) seems kind of backwards. (But I say this knowing nothing
>> >>>> about the tool.)
>> >>>>
>> >>> I don't think we can reliably use the m2p for PV domains because
>> >>> PV domains don't always issue a m2p update hypercall when they change
>> >>> their p2m mapping.
>> >> This only applies to foreign pages which won't be very interesting to a
>> >> crash tool.
>> > True. I think the main reason crash hasn't done this is that it cannot
>> > find the hypervisor maintained m2p list. It should be sufficient to add
>> > some more fields to XEN_VMCOREINFO, so that crash can locate the
>> > mapping in the dump.
>> 
>> The M2P lives at an ABI-specified location in all virtual address spaces
>> for PV guests.
>> 
>> Either 0xF5800000 or 0xFFFF800000000000 depending on bitness.
> 
> Hm, this is nice, but kind of chicken-and-egg problem. A system dump
> contains a snapshot of the machine's RAM. But the addresses you
> mentioned are virtual addresses. How do I translate them to physical
> addresses without an m2p mapping? I need at least the value of CR3 for
> that domain, and most likely a way to determine if it is a PV domain.

This ought to also be present in Xen's master page table
(idle_pg_table[]), and I suppose we can take for granted a symbol
table being available.

Jan
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