On Fri, 2018-07-13 at 19:49 +0200, Sebastian Andrzej Siewior wrote:

In v4.16-RT I noticed a number of warnings from task_fpsimd_load(). The
code disables BH and expects that it is not preemptible. On -RT the
task remains preemptible but remains the same CPU. This may corrupt the
content of the SIMD registers if the task is preempted during
saving/restoring those registers.
Add a locallock around this process. This avoids that the any function
within the locallock block is invoked more than once on the same CPU.

The kernel_neon_begin() can't be kept preemptible. If the task-switch notices
TIF_FOREIGN_FPSTATE then it would restore task's SIMD state and we lose the
state of registers used for in-kernel-work. We would require additional storage
for the in-kernel copy of the registers. But then the NEON-crypto checks for
the need-resched flag so it shouldn't that bad.
The preempt_disable() avoids the context switch while the kernel uses the SIMD
registers. Unfortunately we have to balance out the migrate_disable() counter
because local_lock_bh() is invoked in different context compared to its unlock
counterpart.

__efi_fpsimd_begin() should not use kernel_fpu_begin() due to its
preempt_disable() context and instead save the registers always in its
extra spot on RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---

This seems to make work (crypto chacha20-neon + cyclictest). I have no
EFI so I have no clue if saving SIMD while calling to EFI works.

All is not well on cavium test box.  I'm seeing random errors ala...

./include/linux/fs.h:3137:11: internal compiler error: Segmentation fault
./include/linux/bio.h:175:1: internal compiler error: in grokdeclarator, at c/c-decl.c:7023

...during make -j96 (2*cpus) kbuild.  Turns out 4.14-rt has this issue
as well, which is unsurprising if it's related to fpsimd woes.  Box
does not exhibit the issue with NONRT kernels, PREEMPT or NOPREEMPT.

To file under FWIW, arm64 configured SLE15-RT, 4.12 based kernel
containing virgin @stable arch/arm64/kernel/fpsimd.c, does not exhibit
the problem. (relevant? dunno, it may be unrelated to fpsimd.c).

	-Mike