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

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Message-Id: <CC25C839-1C64-4BD0-818B-4DA0CD7EC28B@arm.com>
Date:	Sun, 31 Aug 2014 23:11:29 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Leo Yan <leoy@...vell.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v1] arm64: fix bug for reloading FPSIMD state after cpu power off

On 31 Aug 2014, at 06:39, Leo Yan <leoy@...vell.com> 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).

Catalin--
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