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

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Message-ID: <5654694D.1060909@citrix.com>
Date:	Tue, 24 Nov 2015 13:42:37 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>, Petr Tesarik <ptesarik@...e.cz>
CC:	Juergen Gross <JGross@...e.com>, <olaf@...fle.de>, <keir@....org>,
	DanielKiper <daniel.kiper@...cle.com>,
	<linux-kernel@...r.kernel.org>, <dslutz@...izon.com>,
	<anderson@...hat.com>, David Vrabel <david.vrabel@...rix.com>,
	David Mair <DMAIR@...e.com>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>,
	"Malcolm Crossley" <malcolm.crossley@...rix.com>,
	<crash-utility@...hat.com>
Subject: Re: [Xen-devel] crash tool - problem with new Xen linear virtual
 mapped sparse p2m list

On 24/11/15 13:41, Andrew Cooper wrote:
> On 24/11/15 13:39, Jan Beulich wrote:
>>>>> 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.
> The idle_pg_table is already present in the VMCORE notes.

Ah, except it is aliased to the name pgd_l4.

~Andrew
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