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

Laszlo Ersek lersek at redhat.com
Wed Nov 12 06:10:10 PST 2014

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.

>> What if the ELF dumper used SHF_COMPRESSED or could dump an ELF.xz?
> Not the same thing. With KDUMP, each page is compressed separately, so
> if a utility like crash needs a page from the middle, it can find it
> and unpack it immediately. If we had an ELF.xz, then the whole file
> must be unpacked before it can be used. And unpacking a few terabytes
> takes ... a while. ;-)
>> How does QEMU figure out the kernel version information?
> Good question. Who can answer this part?

I can.

(Apologies for being a bit non-responsive, I'm swamped. I figured I'd
let the discussion unfold a bit between the kdump experts.)

So, QEMU doesn't figure out the kernel version information. It just
dumps the guest-physical frames, and that's it.

I linked the code before that populates the kdump header. The links and
function names are still visible above.


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