arm64: Memory hotplug support for arm64 platform

This is a second and improved version of the patch previously released
in [3].

It builds on the work by Scott Branden [2] and, henceforth,
it needs to be applied on top of Scott's patches [2].
Comments are very welcome.

Changes from the original patchset and known issues:

- Compared to Scott's original patchset, this work adds the mapping of
  the new hotplugged pages into the kernel page tables. This is done by
  copying the old swapper_pg_dir over a new page, adding the new mappings,
  and then switching to the newly built pg_dir (see `hotplug_paging` in
  arch/arm64/mmu.c). There might be better ways to to this: suggestions
  are more than welcome.

- The stub function for `arch_remove_memory` has been removed for now; we
  are working in parallel on memory hot remove, and we plan to contribute
  it as a separate patch.

- Corresponding Kconfig flags have been added;

- Note that this patch does not work when NUMA is enabled; in fact,
  the function `memory_add_physaddr_to_nid` does not have an
  implementation when the NUMA flag is on: this function is supposed to
  return the nid the hotplugged memory should be associated with. However
  it is not really clear to us  yet what the semantics of this function
  in the context of a NUMA system should be. A quick and dirty fix would
  be to always attach to the first available NUMA node.

- In arch/arm64/mm/init.c `arch_add_memory`, we are doing a hack with the
  nomap memory block flags to satisfy preconditions and postconditions of
  `__add_pages` and postconditions of `arch_add_memory`. Compared to
  memory hotplug implementation for other architectures, the "issue"
  seems to be in the implemenation of `pfn_valid`. Suggestions on how
  to cleanly avoid this hack are welcome.

This patchset can be tested by starting the kernel with the `mem=X` flag, where
X is less than the total available physical memory and has to be multiple of
MIN_MEMORY_BLOCK_SIZE. We also tested it on a customised version of QEMU
capable to emulate physical hotplug on arm64 platform.

To enable the feature the CONFIG_MEMORY_HOTPLUG compilation flag
needs to be set to true. Then, after memory is physically hotplugged,
the standard two steps to make it available (as also documented in
Documentation/memory-hotplug.txt) are:

(1) Notify memory hot-add
 	echo '0xYY000000' > /sys/devices/system/memory/probe

where 0xYY000000 is the first physical address of the new memory section.

(2) Online new memory block(s)
    echo online > /sys/devices/system/memory/memoryXXX/state
    -- or --
    echo online_movable > /sys/devices/system/memory/memoryXXX/state

where XXX corresponds to the ids of newly added blocks.

Onlining can optionally be automatic at hot-add notification by enabling
the global flag:
	echo online > /sys/devices/system/memory/auto_online_blocks
or by setting the corresponding config flag in the kernel build.

Again, any comment is highly appreciated.

[1] https://lkml.org/lkml/2016/11/17/49
[2] https://lkml.org/lkml/2016/12/1/811
[3] https://lkml.org/lkml/2016/12/14/188



Change-Id: I545807e3121c159aaa2f917ea914ee98f38fb296
Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
Patch-mainline: linux-kernel @ 11 Apr 2017, 18:25
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
[arunks@codeaurora.org: fix to pass checker test]
Signed-off-by: Arun KS <arunks@codeaurora.org>
[sudaraja@codeaurora.org: fixed merge conflicts]
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
[swatsrid@codeaurora.org: Fix merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>