[Crash-utility] Re: crash with Xen dom0 image from kdump

[David Anderson]

Horms wrote:

>On Fri, Jun 02, 2006 at 09:28:07AM -0400, Dave Anderson wrote:
>
>>Kazuo Moriwaka wrote:
>>
>>With a xendump, the phys_to_machine_mapping array can be
>>found by following the page tables from any of the cr3 values
>>found in the dump header.
>>
>>It would seem simple enough to have the xen/kdump code store
>>a legitimate dom0 cr3 value somewhere in the ELF header.  Would
>>that be possible to explore?  Note that I'm not really interested
>>in the other guest domains, at least at this point, but I could see
>>it potentially helpful to recreate the crash environment for the
>>guest domains as well, so perhap an array of per-domain cr3
>>values in an ELF header note section would be possible?
>>
>
>I haven't looked into this, but yes I think that should be quite
>possible as I believe that dom0's cr3 is stored in the hypervisor
>somewhere. The main problem (for me) is where to put the extra crash
>note and how to add it without having to modify the user-space kexec
>tool - currently that code does not need to be modified in order to work
>with xen. Would it be enough to have it stored in a symbol in the
>hypervisor (to be honest I don't really understand why crash notes are
>needed at all given that there is a symbol table)? Or alternatively,
>could you give me suggestions on the crash-note front? I'll chase up
>where dom0's cr3 is currently saved.
>

It's conceivable, but highly unlikely, that it can be found in the vmcore in
its current state.  My suggestion of storing at least one cr3 for dom0, and
even better, an array of cr3's for all of the domains, was based upon how
I figure it out now using the xendump format.  It's just that for 
domains with
writable page tables, I at least need a starting point "hook"; and with a
legitimate cr3 (that has the kernel mappings), I can then walk the domain's
page tables (all of which contain mfn's instead of pfn's in the case of 
writable
page table domains).  

(Note that there's some churn going on with the kexec/kdump patch to 
replace its
current manner of over-writing the pgd in use at the dump of the crash 
-- it's
not clear to me whether that would be a problem until such time as that 
kdump
quirk is "fixed".)

But it would also be possible if I could access the domain's 
xen_start_info->mfn_list
value, I could also re-create the list of mfn's that make up the list of 
pages in each
domain's phys_to_machine_mapping array.  The kernel does this, based 
upon what
 its max_pfn value is determined to be:

                phys_to_machine_mapping = alloc_bootmem_low_pages(
                     max_pfn * sizeof(unsigned long));
                memset(phys_to_machine_mapping, ~0,
                       max_pfn * sizeof(unsigned long));
                memcpy(phys_to_machine_mapping,
                       (unsigned long *)xen_start_info->mfn_list,
                       xen_start_info->nr_pages * sizeof(unsigned long));

So my wild guess was that I could possibly find the xen_start_info 
structures
stored in the vmcore as it exists now, but that's probably impossible. 
 I don't
know enough about the hypervisor code.

And again, I don't see why something could *not* be found in the xen symbol
table *now*, but my point was that it unnecessarily adds the burden of 
having to
incorporate that file in the process, instead of simply needing the 
vmlinux and
vmcore file.  Using a PT_NOTE section is simply one manner of passing
arbitraray data back to the ultimate user of the vmcore file.  

Dave
  





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