[RFC PATCH 0/6] ARM: enable IRQ stacks and vmap'ed stacks for UP

[Ard Biesheuvel]

Enable the use of the TLS register to hold the 'current' pointer for all
configurations that can support it, including non-SMP ones that target
v6k or later CPUs, and multi-platform SMP ones that also support v6
based UP systems.

The remaining configurations are all strictly UP, which means we can
switch to a global variable to hold the current pointer. By doing this,
we can enable THREAD_INFO_IN_TASK, which moves thread info off the
stack, protecting it from overflows. It also permits us to enable IRQ
stacks and vmap'ed stacks for UP configurations as well.

Supporting v6 cores without SMP extensions in SMP configurations (e.g.,
omap2plus_defconfig or imx_v6_v7_defconfig) makes this a bit tricky, and
this is a feature we may consider dropping entirely in the future. But
for the time being, we can support this mode as well.

The accesses to the global variable holding 'current' are constructed in
a way that ensures that no literal pool accesses (and associated D-cache
misses) are needed unless the access is from a module and module PLTs
are enabled. This means that accessing 'current' is just as costly as
before, as it used to require some arithmetic involving the stack
pointer and a load from the thread_info::task field.

However, accessing thread_info itself now also involves a load, although
it should be noted that all thread_info and current accesses now go via
the same variable, which is therefore expected to be hot in the caches
at all times.

Cc: Russell King <linux at armlinux.org.uk>
Cc: Nicolas Pitre <nico at fluxnic.net>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Kees Cook <keescook at chromium.org>
Cc: Keith Packard <keithpac at amazon.com>
Cc: Linus Walleij <linus.walleij at linaro.org>
Cc: Nick Desaulniers <ndesaulniers at google.com>
Cc: Tony Lindgren <tony at atomide.com>

Ard Biesheuvel (6):
  ARM: entry: preserve thread_info pointer in switch_to
  ARM: module: implement support for PC-relative group relocations
  ARM: percpu: add SMP_ON_UP support
  ARM: smp: defer TPIDRURO update for SMP v6 configurations too
  ARM: use TLS register for 'current' on !SMP as well
  ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems

 arch/arm/Kconfig                   |  10 +-
 arch/arm/include/asm/assembler.h   | 136 +++++++++++++++-----
 arch/arm/include/asm/current.h     |  56 +++++++-
 arch/arm/include/asm/elf.h         |   3 +
 arch/arm/include/asm/percpu.h      |  29 ++++-
 arch/arm/include/asm/switch_to.h   |   3 +-
 arch/arm/include/asm/thread_info.h |  27 ----
 arch/arm/include/asm/tls.h         |   4 +-
 arch/arm/kernel/asm-offsets.c      |   3 -
 arch/arm/kernel/entry-armv.S       |  26 ++--
 arch/arm/kernel/entry-header.S     |   8 +-
 arch/arm/kernel/head-common.S      |   4 +-
 arch/arm/kernel/module.c           |  63 +++++++++
 arch/arm/kernel/process.c          |   7 +-
 arch/arm/kernel/traps.c            |   4 +
 arch/arm/mm/Kconfig                |   1 +
 16 files changed, 289 insertions(+), 95 deletions(-)

-- 
2.30.2