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Re: [Xen-devel] Re: [PATCH]: kexec: framework and i386

On Sunday 23 April 2006 15:33, Ian Pratt wrote:
> > In summary, for kdump "linux crashdump=64M@16M"
> > becomes "xen kdump_megabytes=64 kdump_megabytes_base=16"
> Please can you explain a bit about how kdump works, and why a physically
> contiguous region with known base address is required. What actually
> gets written out in the crash dump and in what format?

The reserved region is the memory space for the "dump kernel".  I believe the 
base address has to correspond to the base address compiled into the dump 
kernel - since we don't want the dump kernel to try to own all of memory.  
It's native Linux, so it likes to run in contiguous memory.

When a panic occurs, Linux kexec jumps into the preloaded kdump kernel (if 
any).  This kernel then reinitiases the hardware, using its own device 
drivers and uses these to write out the dump to disk.  ISTR that the dump 
format is currently ELF, although I remember some talk on the Fastboot ML 
about adding some extra headers to make OS debugging easier.

It's a nice solution because you don't rely on the hosed kernel to do the dump 
for you, and you don't disturb its state in the process.  It also makes it 
easy to do things like dumping to network devices, etc.

In our case it has the added bonus that on dom0 *or* a Xen crash it ought to 
be possible to kexec into a native Linux kernel which could dump (possibly 
some configurable combination of) Xen itself, dom0, and all the other 
domains.  Admittedly hypervisor crashes / hangs are rare, but it might aid 
debugging to be able to get a reliable dump of a crashed / hung Xen.

This would also integrate with the Linux dump infrastructure, which would be 
useful to have.

> Tim Deegan submitted a patch to add support for multiboot images (such
> as Xen) to kexec a couple of years ago, and I believe it has been part
> of the standard package for some time.

It was in there last time I looked at the source code...  I've never actually 
used it though, so in principle I guess it could have rotted.  Or there could 
just be something weird happenning.


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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