[RFC PATCH v2 0/2] Simplify kernel-mode NEON

[Dave Martin]

This series aims to simplify kernel-mode NEON.

The main motivation for these changes is that supporting kernel-mode
NEON alongside SVE is tricky with the current framework: the current
partial save/restore mechanisms would need additional porting to
save/restore the extended SVE vector bits, and this renders the cost
saving of partial save/restore rather doubtful -- even if not all vector
registers are saved, the save/restore cost will still grow with
increasing vector length.  We could get the mechanics of this to work in
principle, but it doesn't feel like the right thing to do.

If we withdraw kernel-mode NEON support for hardirq context, accept some
extra softirq latency and disable nesting, then we can simplify the code
by always saving the task context directly into task_struct and
deferring all restore to ret_to_user.  Previous discussions with Ard
Biesheuvel suggest that this is a feasible approach and reasonably
aligned with other architectures.

The main impact on client code is that callers must check that
kernel-mode NEON is usable in the current context and fall back to a
non-NEON when necessary.  Ard has already done some work on this. [1]

The interesting changes are all in patch 2: the first patch just adds a
header inclusion guard that I noted was missing.

[1] git://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git kernel-mode-neon


I've only build-tested so far.

Ard, do you have any suggestions for how to test these changes
effectively?

Cheers
---Dave


Dave Martin (2):
  arm64: neon: Add missing header guard in <asm/neon.h>
  arm64: neon: Remove support for nested or hardirq kernel-mode NEON

 arch/arm64/include/asm/fpsimd.h       |  14 -----
 arch/arm64/include/asm/fpsimdmacros.h |  56 -----------------
 arch/arm64/include/asm/neon.h         |  12 +++-
 arch/arm64/include/asm/simd.h         |  56 +++++++++++++++++
 arch/arm64/kernel/entry-fpsimd.S      |  24 -------
 arch/arm64/kernel/fpsimd.c            | 114 ++++++++++++++++++++++++----------
 6 files changed, 146 insertions(+), 130 deletions(-)
 create mode 100644 arch/arm64/include/asm/simd.h

-- 
2.1.4