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Date: Thu, 15 Dec 2016 18:20:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Kyle Huey <khuey@kylehuey.com>, Andy Lutomirski <luto@kernel.org>
Subject: Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra()
Message-ID: <20161215172015.GZ3124@twins.programming.kicks-ass.net>
References: <20161215162648.061449202@linutronix.de>
 <20161215164240.736387680@linutronix.de>
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On Thu, Dec 15, 2016 at 04:44:02PM -0000, Thomas Gleixner wrote:
>  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>  		      struct tss_struct *tss)
>  {
>  	struct thread_struct *prev, *next;
> +	unsigned long tifp, tifn;
>  
>  	prev = &prev_p->thread;
>  	next = &next_p->thread;
>  
> +	tifn = task_thread_info(next_p)->flags;
> +	tifp = task_thread_info(prev_p)->flags;
> +	switch_to_bitmap(tss, prev, next, tifp, tifn);
> +
> +	propagate_user_return_notify(prev_p, next_p);
> +
> +	if ((tifp ^ tifn) & _TIF_BLOCKSTEP) {
>  		unsigned long debugctl = get_debugctlmsr();
>  
>  		debugctl &= ~DEBUGCTLMSR_BTF;
> +		if (tifn & _TIF_BLOCKSTEP)
>  			debugctl |= DEBUGCTLMSR_BTF;
>  		update_debugctlmsr(debugctl);
>  	}

Going by the toggle patter you have elsewhere, wouldn't that then be:

	if ((tifp ^ tifn) & _TIF_BLOCKSTEP) {
		unsigned long debugctl = get_debugctlmsr();

		debugctl ^= DEBUGCTLMSR_BTF;
		update_debugctlmsr(debugctl);
	}

?