On Tue, 2 Mar 2021 10:01:09 +0100, Ard Biesheuvel wrote:

[ TL;DR for the non-ARM folks on CC: disabling softirq processing when using
  SIMD in kernel mode could reduce complexity and improve performance, but we
  need to decide whether we can do this, and how much softirq processing
  latency we can tolerate. If we can find a satisfactory solution for this,
  we might do the same for x86 and 32-bit ARM as well.

  However, based on preliminary off-list discussions with peterz and luto, it
  seems that for x86, there is a preference for using per-CPU buffers to
  preserve/restore the task context's kernel mode SIMD state when the task is
  interrupted to perform kernel mode SIMD in softirq context. On arm64, we
  actually had this arrangement before, and removed it because it made
  reasoning about preserving/restoring userland SVE state (32 SIMD registers
  of up to 2 kbit in size) rather complex. ]

[...]

Applied to arm64 (for-next/neon-softirqs-disabled), thanks!

[1/9] arm64: assembler: remove conditional NEON yield macros
      https://git.kernel.org/arm64/c/27248fe1abb2
[2/9] arm64: assembler: introduce wxN aliases for wN registers
      https://git.kernel.org/arm64/c/4c4dcd3541f8
[3/9] arm64: fpsimd: run kernel mode NEON with softirqs disabled
      https://git.kernel.org/arm64/c/13150149aa6d

-- 
Catalin


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