[PATCH 0/2] kexec_file: Load kernel at top of system RAM if required

[Baoquan He]

Justification:
==============
Kexec_load interface has been doing top down searching and loading
kernel/initrd/purgtory etc to prepare for kexec reboot. In that way,
the benefits are that it avoids to consume and fragment limited low
memory which satisfy DMA buffer allocation and big chunk of continuous
memory during system init; and avoids to stir with BIOS/FW reserved
or occupied areas, or corner case handling/work around/quirk occupied
areas when doing system init. By the way, the top-down searching and
loading of kexec-ed kernel is done in user space utility code.

For kexec_file loading, even if kexec_buf.top_down is 'true', it's simply
ignored. It calls walk_system_ram_res() directly to go through all
resources of System RAM bottom up, to find an available memory region,
then call locate_mem_hole_callback() to allocate memory in that found
memory region from top to down. This is not expected and inconsistent
with kexec_load.

Implementation
===============
In patch 1, introduce a new function walk_system_ram_res_rev() which is
a variant of walk_system_ram_res(), it walks through a list of all the
resources of System RAM in reversed order, i.e., from higher to lower.

In patch 2, check if kexec_buf.top_down is 'true' in kexec_walk_resources(),
if yes, call walk_system_ram_res_rev() to find memory region of system RAM
from top to down to load kernel/initrd etc.

Background information:
=======================
And I ever tried this in the past in a different way, please see below
link. In the post, I tried to adjust struct sibling linking code,
replace the the singly linked list with list_head so that
walk_system_ram_res_rev() can be implemented in a much easier way.
Finally I failed.
https://lore.kernel.org/all/20180718024944.577-4-bhe@redhat.com/

This time, I picked up the patch from AKASHI Takahiro's old post and
made some change to take as the current patch 1:
https://lists.infradead.org/pipermail/linux-arm-kernel/2017-September/531456.html

Testing:
========
Only tried on x86_64

Baoquan He (2):
  resource: add walk_system_ram_res_rev()
  kexec_file: Load kernel at top of system RAM if required

 include/linux/ioport.h |  3 +++
 kernel/kexec_file.c    |  2 ++
 kernel/resource.c      | 61 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+)

-- 
2.41.0