Subject: [PATCH v2 0/3] ARM: v7: get rid of boot time mini stack

From: Ard Biesheuvel <[email-masked]>
Date: Wed, 10 Feb 2021 19:55:29 +0100

The v7 boot code uses a small chunk of BSS to preserve some register
contents across a call to v7_invalidate_l1 that occurs with the MMU and
caches disabled. Memory accesses in such cases are tricky on v7+, given
that the architecture permits some unintuitive behaviors (it is
implementation defined whether accesses done with the MMU and caches off
may hit in the caches). Also, cache invalidation is not safe under
virtualization if the intent is to retain stores issued directly to DRAM,
as the hypervisor may upgrade invalidate operations to clean+invalidate,
resulting in DRAM contents to be overwritte by the dirty cachelines that
we were trying to evict in the first place.

So let's address this issue, by removing the need for this stack to
exist in the first place: v7_invalidate_l1 can be rewritten to use fewer
registers, which means fewer registers need to be preserved, and we have
enough spare registers available.

Patch #1 adds a missing ISB. This patch is included separately so it can
be backported if desired.

Patch #2 rewrites v7_invalidate_l1 so it only uses 5 registers (not
counting lr which it must preserve as well)

Patch #3 updates the callers to use spare registers instead of the mini
stack to stash the values that need to be preserved across the calls to
v7_invalidate_l1.

Changes since v1:
- use correct stop condition in outer loop (cc not mi)
- bring back ENDPROC() in patch #3
- add Nico's ack

This version of the series was build/boot tested here:
https://kernelci.org/build/ardb/branch/for-kernelci/kernel/v5.11-rc7-3-gafc656039a99/
https://kernelci.org/test/job/ardb/branch/for-kernelci/kernel/v5.11-rc7-3-gafc656039a99/plan/baseline/

Cc: Marc Zyngier <[email-masked]>
Cc: Russell King <[email-masked]>
Cc: Linus Walleij <[email-masked]>
Cc: Nicolas Pitre <[email-masked]>

Ard Biesheuvel (3):
  ARM: cache-v7: add missing ISB after cache level selection
  ARM: cache-v7: refactor v7_invalidate_l1 to avoid clobbering r5/r6
  ARM: cache-v7: get rid of mini-stack

 arch/arm/include/asm/memory.h | 15 -----
 arch/arm/mm/cache-v7.S        | 58 ++++++++++----------
 arch/arm/mm/proc-v7.S         | 39 ++++++-------
 3 files changed, 47 insertions(+), 65 deletions(-)

-- 
2.30.0

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