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

[Bhupesh SHARMA]

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'.

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
>
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