[PATCH v2 0/2] ARM: move FDT mapping out of linear region

[Ard Biesheuvel <ardb@kernel.org>]

For the ARM kernel, setting up the kernel's virtual address space at boot
is tricky, given the risk of collision between 1:1 mapped regions and
virtually remapped regions. There is also a concern regarding the exact
mapping attributes that are needed for each region, which differs between
ARM architecture revisions, UP vs SMP etc

For this reason, the kernel VA space is set up in two stages: at early
boot, only the kernel itself and the DT are mapped using section mappings
and later on, all existing mappings are torn down except the one covering
the first memblock covering the kernel, and remapped again using the full
fledged mapping routines that can map at page granularity and use all the
right mapping attributes.

There are cases where this may result in the DT getting unmapped at
this point, and not remapped again. For instance, if the first memblock
is not PMD aligned, we will align it up, and mark the memory below it
as MEMBLOCK_NOMAP, to avoid having to allocate page tables to create
the mapping before we have a mapped memblock to allocate from. If the
DT happens to reside in this region, it will not be mapped at all when
the permanent kernel VA mappings are in place, resulting in crashes.

As we happen to have a 4 MB hole in the kernel VA space (between the
end of the VMALLOC space and the start of the FIXMAP region), let's
use it to create a permanent, read-only mapping of the DT that is not
affected by any such issues.

Changes since v1:
- bump the start of the fixmap region by 512 KB so we still have a full
  guard region even on LPAE builds, which will use the entire 4 MB window
  for the read-only mapping of the DT
- update Documentation/arm/memory.rst accordingly
- add Linus's ack

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Rob Herring <robh+dt@kernel.org>

Ard Biesheuvel (2):
  ARM: centralize phys-to-virt conversion of DT/ATAGS address
  ARM: move device tree mapping out of linear region

 Documentation/arm/memory.rst  |  7 ++++++-
 arch/arm/include/asm/fixmap.h |  2 +-
 arch/arm/include/asm/memory.h |  5 +++++
 arch/arm/include/asm/prom.h   |  4 ++--
 arch/arm/kernel/atags.h       |  4 ++--
 arch/arm/kernel/atags_parse.c |  6 +++---
 arch/arm/kernel/devtree.c     |  6 +++---
 arch/arm/kernel/head.S        |  5 ++---
 arch/arm/kernel/setup.c       | 19 ++++++++++++++-----
 arch/arm/mm/init.c            |  1 -
 arch/arm/mm/mmu.c             | 20 ++++++++++++++------
 arch/arm/mm/pv-fixup-asm.S    |  4 ++--
 12 files changed, 54 insertions(+), 29 deletions(-)

-- 
2.17.1


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