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 13:57:01 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	Malcolm Crossley <malcolm.crossley@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	Juergen Gross <JGross@...e.com>, <olaf@...fle.de>,
	<keir@....org>, <linux-kernel@...r.kernel.org>,
	<dslutz@...izon.com>, <anderson@...hat.com>,
	David Mair <DMAIR@...e.com>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <crash-utility@...hat.com>
Subject: Re: [Xen-devel] crash tool - problem with new Xen linear virtual
 mapped sparse p2m list

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.

Petr T
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