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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.11.15 at 13:57, <ptesarik@xxxxxxx> wrote:
> V Tue, 24 Nov 2015 10:35:03 +0000
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> napsÃno:
> 
>> On 24/11/15 10:17, Petr Tesarik wrote:
>> > On Tue, 24 Nov 2015 10:09:01 +0000
>> > David Vrabel <david.vrabel@xxxxxxxxxx> 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@xxxxxxxx> 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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