Re: [Qemu-devel] uniquely identifying KDUMP files that originate from QEMU

[Petr Tesarik <ptesarik@suse.cz>]

On Tue, 11 Nov 2014 12:27:44 -0500
Christopher Covington <cov@codeaurora.org> wrote:

> On 11/11/2014 06:22 AM, Laszlo Ersek wrote:
> > (Note: I'm not subscribed to either qemu-devel or the kexec list; please
> > keep me CC'd.)
> > 
> > QEMU is able to dump the guest's memory in KDUMP format (kdump-zlib,
> > kdump-lzo, kdump-snappy) with the "dump-guest-memory" QMP command.
> > 
> > The resultant vmcore is usually analyzed with the "crash" utility.
> > 
> > The original tool producing such files is kdump. Unlike the procedure
> > performed by QEMU, kdump runs from *within* the guest (under a kexec'd
> > kdump kernel), and has more information about the original guest kernel
> > state (which is being dumped) than QEMU. To QEMU, the guest kernel state
> > is opaque.
> > 
> > For this reason, the kdump preparation logic in QEMU hardcodes a number
> > of fields in the kdump header. The direct issue is the "phys_base"
> > field. Refer to dump.c, functions create_header32(), create_header64(),
> > and "include/sysemu/dump.h", macro PHYS_BASE (with the replacement text
> > "0").
> > 
> > http://git.qemu.org/?p=qemu.git;a=blob;f=dump.c;h=9c7dad8f865af3b778589dd0847e450ba9a75b9d;hb=HEAD
> > 
> > http://git.qemu.org/?p=qemu.git;a=blob;f=include/sysemu/dump.h;h=7e4ec5c7d96fb39c943d970d1683aa2dc171c933;hb=HEAD
> > 
> > This works in most cases, because the guest Linux kernel indeed tends to
> > be loaded at guest-phys address 0. However, when the guest Linux kernel
> > is booted on top of OVMF (which has a somewhat unusual UEFI memory map),
> > then the guest Linux kernel is loaded at 16MB, thereby getting out of
> > sync with the phys_base=0 setting visible in the KDUMP header.
> > 
> > This trips up the "crash" utility.
> > 
> > Dave worked around the issue in "crash" for ELF format dumps -- "crash"
> > can identify QEMU as the originator of the vmcore by finding the QEMU
> > notes in the ELF vmcore. If those are present, then "crash" employs a
> > heuristic, probing for a phys_base up to 32MB, in 1MB steps.
> 
> What advantages does KDUMP have over ELF?

It's smaller (data is compressed), and it contains a header with some
useful information (e.g. the crashed kernel's version and release).

HTH,
Petr T