[BUG] dm-crypt broken after 2632e2521769 ("arm64: fpsimd: Implement lazy restore for kernel mode FPSIMD")

[Will Deacon]

Hi Johannes,

On Tue, May 21, 2024 at 08:22:08AM +0200, Johannes Nixdorf wrote:
> Bad news: I hit the bug again with 2632e2521769 ("arm64: fpsimd: Implement
> lazy restore for kernel mode FPSIMD") reverted during prolonged interactive
> usage with the downstream Asahi Linux kernel.

Damn, but thanks for the update. I have to ask, but are you absolutely
sure this was with 2632e2521769 reverted? If you're able to double-check
that, it would be great, since we're having trouble reproducing the
issue.

> This prompted me to adjust the reproducer to be closer to the desktop use
> case, which then also found aefbab8e77eb ("arm64: fpsimd: Preserve/restore
> kernel mode NEON at context switch"). With the vanilla kernel before the
> commit or that commit reverted on the Asahi Linux kernel the new reproducer
> also sees no bug, and interactive usage seems fine.

I've already reverted 2632e2521769 ("arm64: fpsimd: Implement lazy
restore for kernel mode FPSIMD"), so it sounds like I should revert
aefbab8e77eb ("arm64: fpsimd: Preserve/restore kernel mode NEON at
context switch") as well while we work to reproduce the issue.

Will