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

[Petr Tesarik]

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).
> 
> 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?

Petr T