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 13:46:10 +0000
From:	Ian Campbell <ian.campbell@...rix.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Petr Tesarik <ptesarik@...e.cz>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Juergen Gross <JGross@...e.com>, <olaf@...fle.de>, <keir@....org>,
	"David Mair" <DMAIR@...e.com>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	<linux-kernel@...r.kernel.org>, <dslutz@...izon.com>,
	<anderson@...hat.com>, Jan Beulich <JBeulich@...e.com>,
	<xen-devel@...ts.xenproject.org>,
	"Malcolm Crossley" <malcolm.crossley@...rix.com>,
	<boris.ostrovsky@...cle.com>, <crash-utility@...hat.com>
Subject: Re: [Xen-devel] crash tool - problem with new Xen linear virtual
 mapped sparse p2m list

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.

Ian.
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