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

Dave Young dyoung at redhat.com
Thu Oct 29 00:01:08 PDT 2015

On 10/29/15 at 03:53pm, AKASHI Takahiro wrote:
> On 10/29/2015 03:40 PM, Dave Young wrote:
> >
> >On 10/29/15 at 02:55pm, AKASHI Takahiro wrote:
> >>Dave,
> >>
> >>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.)
> >
> >Hmm, I did not talked about the eflcorehdr=, I means the code to reserve the
> >memory ranges elfcorehdr is using.
> So how does it relate to "mem=" issue?

It is just an example that kexec can pass it along with the usable mem range to
kernel via some interface like dtb blob or some other interfaces. 

> -Takahiro AKASHI
> >Thanks
> >Dave
> >
> >>
> >>-Takahiro AKASHI
> >>
> >>>Thanks
> >>>Dave
> >>>

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