uniquely identifying KDUMP files that originate from QEMU

HATAYAMA Daisuke d.hatayama at jp.fujitsu.com
Tue Nov 11 19:08:38 PST 2014

From: Petr Tesarik <ptesarik at suse.cz>
Subject: Re: uniquely identifying KDUMP files that originate from QEMU
Date: Tue, 11 Nov 2014 13:09:13 +0100

> On Tue, 11 Nov 2014 12:22:52 +0100
> Laszlo Ersek <lersek at redhat.com> 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.
>> Alas, the QEMU notes are not present in the KDUMP-format vmcores that
>> QEMU produces (they cannot be),
> Why? Since KDUMP format version 4, the complete ELF notes can be stored
> in the file (see offset_note, size_note fields in the sub-header).

Yes, the QEMU notes is present in kdump-compressed format. But
phys_base cannot be calculated only from qemu-side. We cannot do more
than the efforts crash utility does for workaround. So, the phys_base
value in kdump-sub header is now designed to have 0 now.

Anyway, phys_base is kernel information. To make it available for qemu
side, there's need to prepare a mechanism for qemu to have any access
to it.

One ad-hoc but simple way is to put phys_base value as part of
VMCOREINFO note information on kernel.

Although there has already been a similar one in VMCOREINFO, like

void arch_crash_save_vmcoreinfo(void)
        VMCOREINFO_SYMBOL(phys_base); <---- This


this is meangless, because this value is a virtual address assigned to
phys_base symbol. To refer to the value of phys_base itself, we need
the phys_base value we are about to get now.

So, instead, if we change this to save the value, not value of symbol
phys_base, we can get phys_base from the VMCOREINFO.

The VMCOREINFO consists simply of string. So it's easy to search
vmcore for it e.g. using strings and grep like this:

$ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100
SYMBOL(phys_base)=ffffffff818e5010  <-- though this is address of phys_base now...

This should also be useful to get phys_base of 2nd kernel, which is
inherently relocated kernel from a vmcore generated using qemu dump.

This is far from well-designed from qemu's point of view, but it would
be manually easier to get phys_base than now.

Obviously, the VMCOREINFO is available only if CONFIG_KEXEC is
enabled. Other users cannot use this.


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