Re: [PATCH v1] arm64: fix bug for reloading FPSIMD state after cpu power off

From: Catalin Marinas
Date: Sun Aug 31 2014 - 18:11:48 EST

On 31 Aug 2014, at 06:39, Leo Yan <leoy@xxxxxxxxxxx> wrote:
> Now arm64 defers reloading FPSIMD state, but this optimization also
> introduces the bug after cpu resume back from low power mode.

You are right, I can see a bug here.

> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index ad8aebb..4caef93 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -268,13 +268,9 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
> {
> switch (cmd) {
> case CPU_PM_ENTER:
> - if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
> - fpsimd_save_state(&current->thread.fpsimd_state);

That’s needed if we enter a low power state directly from a task with
a valid mm (user). I’m not sure that’s possible, but just in case.

> + this_cpu_write(fpsimd_last_state, NULL);

That’s correct. In most cases, we enter low power state from the idle
thread which does not have an mm, so the CPU_PM_EXIT case wouldn’t set
a TIF_FOREIGN_FPSTATE, so thread switching would not detect the change.

> break;
> case CPU_PM_EXIT:
> - if (current->mm)
> - set_thread_flag(TIF_FOREIGN_FPSTATE);

See above for why this may still be needed (it case it returns to user
directly without fpsimd_thread_switch() to detect fpsimd_last_state).

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