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

[Dave Young]

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. 

Thanks
Dave