[PATCH 0/3] arm64: Run kernel mode NEON with preemption enabled

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb at kernel.org>

Currently, kernel mode NEON (SIMD) support is implemented in a way that
requires preemption to be disabled while the SIMD registers are live.
The reason for this is that those registers are not in the set that is
preserved/restored on exception entry/exit and context switch, as this
would impact performance generally, even for workloads where kernel mode
SIMD is not the bottleneck.

However, doing substantial work with preemption disabled is not great,
as it affects scheduling latency, which is especially problematic for
real-time use cases. So ideally, we should keep preemption enabled when
we can, and find another way to ensure that this does not corrupt the
NEON register state of in-kernel SIMD users.

This series implements a suggestion by Mark Rutland, and introduces a
thread_info flag TIF_USING_KMODE_NEON, which indicates to the thread
switch machinery that the task in question has live kernel mode SIMD
state which needs to be preserved and restored. The space needed for
this is allocated in thread_struct. (*)

Given that currently, we run kernel mode NEON with softirqs disabled (to
avoid the need for preserving kernel mode NEON context belonging to task
context while the SIMD unit is being used by code running in softirq
context), just removing the preempt_disable/enable calls is not
sufficient, and we also need to leave softirqs enabled. This means that
we may need to preserve kernel mode NEON state not only on a context
switch, but also when code running in softirq context takes ownership of
the SIMD unit, but this is straight-forward once we add the scratch
space to thread_struct.

(*) We might decide to allocate this space (~512 bytes) dynamically, if
the thread_struct memory footprint causes issues. However, we should
also explore doing the same for the user space FPSIMD state, as kernel
threads never return to user space and have no need for this allocation.

Cc: Marc Zyngier <maz at kernel.org>
Cc: Will Deacon <will at kernel.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Mark Brown <broonie at kernel.org>

Ard Biesheuvel (3):
  arm64: fpsimd: Drop unneeded 'busy' flag
  arm64: fpsimd: preserve/restore kernel mode NEON at context switch
  arm64: crypto: remove conditional yield logic

 arch/arm64/crypto/aes-glue.c         |  21 ++--
 arch/arm64/crypto/aes-modes.S        |   2 -
 arch/arm64/crypto/sha1-ce-core.S     |   2 -
 arch/arm64/crypto/sha1-ce-glue.c     |  19 +--
 arch/arm64/crypto/sha2-ce-core.S     |   2 -
 arch/arm64/crypto/sha2-ce-glue.c     |  19 +--
 arch/arm64/crypto/sha3-ce-core.S     |   4 +-
 arch/arm64/crypto/sha3-ce-glue.c     |  14 +--
 arch/arm64/crypto/sha512-ce-core.S   |   2 -
 arch/arm64/crypto/sha512-ce-glue.c   |  16 +--
 arch/arm64/include/asm/assembler.h   |  29 -----
 arch/arm64/include/asm/processor.h   |   2 +
 arch/arm64/include/asm/simd.h        |  11 +-
 arch/arm64/include/asm/thread_info.h |   1 +
 arch/arm64/kernel/asm-offsets.c      |   4 -
 arch/arm64/kernel/fpsimd.c           | 128 +++++++++++---------
 16 files changed, 106 insertions(+), 170 deletions(-)

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2.43.0.rc1.413.gea7ed67945-goog