(subset) Re: [PATCH v2 0/9] arm64: rework NEON yielding to avoid scheduling from asm code

On Wed, 3 Feb 2021 12:36:17 +0100, Ard Biesheuvel wrote:

Given how kernel mode NEON code disables preemption (to ensure that the
FP/SIMD register state is protected without having to context switch it),
we need to take care not to let those algorithms operate on unbounded
input data, or we may end up with excessive scheduling blackouts on
CONFIG_PREEMPT kernels.

This is currently handled by the cond_yield_neon macros, which check the
preempt count and the TIF_NEED_RESCHED flag from assembler code, and call
into kernel_neon_end()+kernel_neon_begin(), triggering a reschedule.
This works as expected, but is a bit messy, given how much of the state
preserve/restore code in the algorithm needs to be duplicated, as well as
causing the need to manage the stack frame explicitly. All of this is better
handled by the compiler, especially now that we have enabled features such
as the shadow call stack and BTI, and are working to improve call stack
validation.

[...]

Applied first patch only to arm64 (for-next/crypto), thanks!

[1/9] arm64: assembler: add cond_yield macro
      https://git.kernel.org/arm64/c/d13c613f136c

This is the only patch on the branch, so I'm happy for it to be pulled
into the crypto tree too if it enables some of the other patches to land
in 5.12.

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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