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

Christopher Covington cov at codeaurora.org
Wed Nov 12 05:18:04 PST 2014

On 11/12/2014 03:05 AM, Petr Tesarik wrote:
> On Tue, 11 Nov 2014 12:27:44 -0500
> Christopher Covington <cov at 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).

What if the ELF dumper used SHF_COMPRESSED or could dump an ELF.xz?

How does QEMU figure out the kernel version information?


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