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

Christopher Covington cov at codeaurora.org
Wed Nov 12 07:43:59 PST 2014

On 11/12/2014 10:03 AM, Laszlo Ersek wrote:
> 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.

My newbie understanding is that SHT_NOBITS is the section type of the .bss
section in an everyday executable. It's what makes the section only need a
header table entry in the ELF file, and not sh_size (Size) worth of zeros. The
ELF loader will essentially zero memory beginning at sh_addr (Address) for
sh_size (Size).




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