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

Laszlo Ersek lersek at redhat.com
Wed Nov 12 07:03:11 PST 2014

On 11/12/14 15:48, Christopher Covington wrote:
> Thanks Petr and Laszlo for entertaining my questions. I've got one last one if
> you have the time.
> On 11/12/2014 09:10 AM, Laszlo Ersek wrote:
>> On 11/12/14 14:26, Petr Tesarik wrote:
>>> On Wed, 12 Nov 2014 08:18:04 -0500
>>> Christopher Covington <cov at codeaurora.org> wrote:
>>>> 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).
>> Another advantage is that all zero-filled pages are represented in the
>> kdump file by one shared zero page.
>> The difference in speed of dumping is stunning.
> Would you expect using SHT_NOBITS to give a similar speedup to the ELF dumper?

Sorry, I don't know what SHT_NOBITS is.


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