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Date: Thu, 29 Jan 2015 22:21:23 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] x86,fpu: merge save_init_fpu & unlazy_fpu
Message-ID: <20150129212123.GA31892@redhat.com>
References: <54C2A245.4010307@redhat.com> <20150124202021.GA1285@redhat.com> <54C6CD64.10208@redhat.com> <20150127194030.GA29879@redhat.com> <54C7F4BB.5020509@redhat.com> <20150129204534.GA30530@redhat.com> <20150129160017.6758dd40@cuia.bos.redhat.com>
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On 01/29, Rik van Riel wrote:
>
> The functions save_init_fpu and unlazy_fpu do essentially the
> same thing:

Yes ;) Could you look at 1-3 I sent ?

> Callers of init_fpu do want __thread_fpu_end, so move the call to
> __thread_fpu_end into init_fpu.

I don't think so... Contrary, I think this __thread_fpu_end() is simply
wrong.

>  static inline void save_init_fpu(struct task_struct *tsk)
>  {
> -	WARN_ON_ONCE(!__thread_has_fpu(tsk));
> -
> -	if (use_eager_fpu()) {
> -		__save_fpu(tsk);
> -		return;
> -	}
> -
>  	preempt_disable();
> -	__save_init_fpu(tsk);
> -	__thread_fpu_end(tsk);
> +	if (__thread_has_fpu(tsk)) {
> +		if (use_eager_fpu())
> +			__save_fpu(tsk);
> +		else
> +			__save_init_fpu(tsk);

See the changelog in 2/3. I think we still need __thread_fpu_end() if
__save_init_fpu() returns 0. In this case (_I think_) the state of FPU
doesn't match the saved state. IOW, "save_init" == "save" + "init" (I guess),
and that "init" can (say) reset some control register to default value.


> +	} else if (!use_eager_fpu())
> +		tsk->thread.fpu_counter = 0;

See 1/3, I think this should be simply removed.

> @@ -245,8 +233,10 @@ int init_fpu(struct task_struct *tsk)
>  	int ret;
>  
>  	if (tsk_used_math(tsk)) {
> -		if (cpu_has_fpu && tsk == current)
> -			unlazy_fpu(tsk);
> +		if (cpu_has_fpu && tsk == current) {
> +			save_init_fpu(tsk);
> +			__thread_fpu_end(tsk);

See above.

Oleg.