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 06:55:40 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ian Campbell" <ian.campbell@...rix.com>,
	"Petr Tesarik" <ptesarik@...e.cz>
Cc:	<olaf@...fle.de>, "Andrew Cooper" <andrew.cooper3@...rix.com>,
	"David Vrabel" <david.vrabel@...rix.com>,
	"MalcolmCrossley" <malcolm.crossley@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>,
	"Daniel Kiper" <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 14:46, <ian.campbell@...rix.com> wrote:
> On Tue, 2015-11-24 at 10:35 +0000, Andrew Cooper wrote:
>> 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.
> 
> In theory it can actually be dynamic. XENMEM_machphys_mapping is the way to
> get at it (for both bitnesses).
> 
> For 64-bit guests I think that is most an "in theory" thing and it never
> has actually been so.
> 
> For a 32-bit guest case I don't recall if it is just a 32on32 vs 32on64
> thing, or if something (either guest or toolstack) gets to pick more
> dynamically or even if it is a dom0 vs domU thing.

It's only for 32-on-64 where this range can change (and there it's the
64-bit address that crash would care about anyway).

Jan

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