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

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Date:	Mon, 1 Sep 2014 09:47:05 +0800
From:	Leo Yan <leoy@...vell.com>
To:	Catalin Marinas <catalin.marinas@....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 09/01/2014 06:11 AM, Catalin Marinas wrote:
> 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.

u are right, in some special cases (such like IKS, etc) a normal thread 
also may call cpu_suspend() function so that it will run here; i will 
commit another patch to reserve this part.

>> +		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).

Will reserve this part as well.

Thanks,
Leo Yan
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