[PATCH 13/16] arm64: kdump: add kdump support

AKASHI Takahiro takahiro.akashi at linaro.org
Wed Oct 28 22:55:06 PDT 2015


On 10/23/2015 06:50 PM, Dave Young wrote:
> On 10/22/15 at 06:57pm, AKASHI Takahiro wrote:
>> (added Ard to Cc.)
>> On 10/22/2015 02:15 PM, Dave Young wrote:
>>> On 10/22/15 at 01:29pm, AKASHI Takahiro wrote:
>>>> Hi Dave,
>>>> Thank you for your comment.
>>>> On 10/22/2015 12:25 PM, Dave Young wrote:
>>>>> Hi, AKASHI,
>>>>> On 10/19/15 at 11:38pm, Geoff Levand wrote:
>>>>>> From: AKASHI Takahiro <takahiro.akashi at linaro.org>
>>>>>> On crash dump kernel, all the information about primary kernel's core
>>>>>> image is available in elf core header specified by "elfcorehdr=" boot
>>>>>> parameter. reserve_elfcorehdr() will set aside the region to avoid any
>>>>>> corruption by crash dump kernel.
>>>>>> Crash dump kernel will access the system memory of primary kernel via
>>>>>> copy_oldmem_page(), which reads one page by ioremap'ing it since it does
>>>>>> not reside in linear mapping on crash dump kernel.
>>>>>> Please note that we should add "mem=X[MG]" boot parameter to limit the
>>>>>> memory size and avoid the following assertion at ioremap():
>>>>>> 	if (WARN_ON(pfn_valid(__phys_to_pfn(phys_addr))))
>>>>>> 		return NULL;
>>>>>> when accessing any pages beyond the usable memories of crash dump kernel.
>>>>> How does kexec-tools pass usable memory ranges to kernel? using dtb?
>>>>> Passing an extra mem=X sounds odd in the design. Kdump kernel should get
>>>>> usable ranges and hanle the limit better than depending on an extern kernel
>>>>> param.
>>>> Well, regarding "depending on an external kernel param,"
>>>> - this limitation ("mem=") is compatible with arm(32) implementation although
>>>>    it is not clearly described in kernel's Documentation/kdump/kdump.txt.
>>>> - "elfcorehdr" kernel parameter is mandatory on x86 as well as on arm/arm64.
>>>>    The parameter is explicitly generated and added by kexec-tools.
>>>> Do I miss your point?
>>> Arm previously use atag_mem tag for memory kernel uses, with dtb, Booting.txt
>>> says: The boot loader must pass at a minimum the size and location of the
>>> system memory
>>> In arm64 booting.txt, it does mentions about dtb but without above sentence.
>>> So if you are using dtb to pass memory I think the extra mem= should be not
>>> necessary unless there's other limitations dtb can not been used.
>> I would expect comments from arm64 maintainers here.
>> In my old implementation, I added "usablemem" attributes, along with "reg," to
>> "memory" nodes in dtb to specify the usable memory region on crash dump kernel.
>> But I removed this feature partly because, on uefi system, uefi might pass
>> no memory information in dtb.
> If this is the case there must be somewhere else one can pass memory infomation
> to kernel, the booting.txt should be updated?
> kexec as a boot loader need use same method as the 1st kernel boot loader.
>>> One thing I'm confused is mem= only pass the memory size, where does you pass
>>> the start addresses?
>> In the current arm64 implementation, any regions below the start address will
>> be ignored as system ram.
>>> What if there's multiple sections such as some reserved
>>> ranges 2nd kernel also need?
>> My patch utilizes only a single contiguous region of memory as system ram.
>> One exception that I notice is uefi's runtime data. They will be ioremap'ed separately.
>> Please let me know if there is any other case that should be supported.
> For example the elf headers range, you reserved them in kdump kernel code,
> but kexec-tools can do that early if it can provides all memory info to 2nd
> kernel. Ditto for mark all the memory ranges 1st kernel used as reserved.

It seems to me that the issue you mentioned here is totally independent
from "mem=" issue, isn't it?
(and "elfcorehdr=" is a common way for crash dump kernel to know the region.)

-Takahiro AKASHI

> Thanks
> Dave

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