linux-kernel - Re: [RFC PATCH 05/11] x86,fpu: ensure FPU state is reloaded from memory if task is traced

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Message-ID: <20150113165043.GA26931@redhat.com>
Date:	Tue, 13 Jan 2015 17:50:43 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [RFC PATCH 05/11] x86,fpu: ensure FPU state is reloaded from
	memory if task is traced

On 01/13, Rik van Riel wrote:
>
> >> @@ -412,8 +412,14 @@ static inline void switch_fpu_prepare(struct
> >> task_struct *old, struct task_struc bool preload =
> >> tsk_used_math(new) && (use_eager_fpu() || new->thread.fpu_counter
> >> > 5); if (__thread_has_fpu(old)) { -		if (!__save_init_fpu(old)) 
> >> +		/* +		 * Make sure the FPU state is restored from memory next
> >> time, +		 * if the task has an FPU exception pending, or the
> >> task's in +		 * memory FPU state could be changed by a debugger. 
> >> +		 */ +		if (!__save_init_fpu(old) ||
> >> task_is_stopped_or_traced(old)) cpu = ~0;
> >
> > Well, if debugger wants to change FPU state, it should call
> > init_fpu() which resets .last_cpu ?
>
> Does the ptrace (and utrace, and ... ) code actually do that?

Yes, see xfpregs_get/set. So I think this change is not needed (but I
didn't look at the next patches).

Oleg.

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