[PATCH] Documentation: kdump: describe jumping to dump-capture kernel

[Gi-Oh Kim]

On Wed, Feb 14, 2018 at 10:17 AM, Bhupesh SHARMA
<bhupesh.linux at gmail.com> wrote:
> On Wed, Feb 14, 2018 at 2:32 PM, Gi-Oh Kim <gi-oh.kim at profitbricks.com> wrote:
>> On Wed, Feb 14, 2018 at 2:43 AM, Dave Young <dyoung at redhat.com> wrote:
>>> Hi,
>>> On 02/13/18 at 04:22pm, Gioh Kim wrote:
>>>> Jumping between the system kernel and the dump-capture kernel
>>>> has been supported for long time but there is no description
>>>> how to use it. This patch adds the description how to use kexec tool
>>>> to jump to the dump-capture kernel and jump back to the system kernel.
>>>
>>> I do not think this should belong to kdump documentation.  There are a
>>> lot of choices after a vmcore saving, one can reboot, halt, even go
>>> ahead with real root filesystem init path.  We do not need to document
>>> all these in kdump.txt.
>>>
>>> Since it is a general use case not only for kdump, add more info in
>>> kexec man page would be better.
>>
>> Hi,
>>
>> Whenever I find a kernel option, I search Documentation files, and
>> next is google.
>> So I think it would be better to describe how to use a kernel option
>> or kernel feature in kernel document.
>
> But this is not the kernel documentation for kexec warm reboot.
> Instead this is the kdump documentation. The example you quoted above
> is the 'kexec -l' use case and not 'kexec -p'.


I thought enabling CONFIG_KEXEC_JUMP in kernel is the main topic
and describing 'kexec -l' is showing how to use the CONFIG_KEXEC_JUMP option.
Ok, I'll check the manual page of kexec.
I think it would be enough if I add a short note for -l and
--load-preserve-context about the kernel option.

Thank you ;-)


>
> Rather the man page for kexec-tools should be a good place to include
> this documentation.
>
> Regards,
> Bhupesh
>
>>>
>>>>
>>>> Signed-off-by: Gioh Kim <gi-oh.kim at profitbricks.com>
>>>> ---
>>>>  Documentation/kdump/kdump.txt | 38 ++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 38 insertions(+)
>>>>
>>>> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
>>>> index 51814450a7f8..35b71fef5d88 100644
>>>> --- a/Documentation/kdump/kdump.txt
>>>> +++ b/Documentation/kdump/kdump.txt
>>>> @@ -460,6 +460,44 @@ and the system will boot into the dump-capture kernel.
>>>>  For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
>>>>  "echo c > /proc/sysrq-trigger" or write a module to force the panic.
>>>>
>>>> +Jump between the System kernel and the Dump-capture kernel
>>>> +===============================
>>>> +
>>>> +Without system crash, the system can jump to the dump-capture kernel.
>>>> +
>>>> +1) Enable "jump between system kernel and dump-capture kernel" support under
>>>> +   "Processor type and features"
>>>> +
>>>> +   CONFIG_KEXEC_JUMP=y
>>>> +
>>>> +2) Load the dump-capture kernel with --load-preserve-context and mem-max
>>>> +   options as following.
>>>> +
>>>> +   kexec -l <dump-capture-kernel-vmlinux-image> \
>>>> +   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
>>>> +   --append="root=<root-dev> <arch-specific-options>" \
>>>> +   --load-preserve-context \
>>>> +   --mem-max=<the highest memory address addr to load code into>
>>>> +
>>>> +3) Jump to the loaded kernel
>>>> +
>>>> +   kexec -e
>>>> +
>>>> +Now the system is running with the dump-capture kernel. You can jump back
>>>> +to the system kernel.
>>>> +
>>>> +1) Find kexec_jump_back_entry address in kernel booting parameters in
>>>> +   /proc/cmdline. That is the address for kexec to jump to. For example:
>>>> +   kexec_jump_back_entry=0x00000000000810d2
>>>> +
>>>> +2) Following command sets the jump-back address for kexec.
>>>> +
>>>> +   kexec --load-jump-back-helper --entry=0x810d2
>>>> +
>>>> +3) Jump to the system kernel
>>>> +
>>>> +   kexec -e
>>>> +
>>>>  Write Out the Dump File
>>>>  =======================
>>>>
>>>> --
>>>> 2.11.0
>>>>
>>>
>>> Thanks
>>> Dave
>>
>>
>>
>> --
>> GIOH KIM
>> Linux Kernel Entwickler
>>
>> ProfitBricks GmbH
>> Greifswalder Str. 207
>> D - 10405 Berlin
>>
>> Tel:       +49 176 2697 8962
>> Fax:      +49 30 577 008 299
>> Email:    gi-oh.kim at profitbricks.com
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>>
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>> Geschäftsführer: Achim Weiss, Matthias Steinberg



-- 
GIOH KIM
Linux Kernel Entwickler

ProfitBricks GmbH
Greifswalder Str. 207
D - 10405 Berlin

Tel:       +49 176 2697 8962
Fax:      +49 30 577 008 299
Email:    gi-oh.kim at profitbricks.com
URL:      https://www.profitbricks.de

Sitz der Gesellschaft: Berlin
Registergericht: Amtsgericht Charlottenburg, HRB 125506 B
Geschäftsführer: Achim Weiss, Matthias Steinberg