linux-kernel - Re: question about save_xstate_sig()

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Date:	Tue, 27 Jan 2015 15:50:17 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

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On 01/27/2015 03:27 PM, Rik van Riel wrote:
> On 01/27/2015 02:40 PM, Oleg Nesterov wrote:

>>>> - Why unlazy_fpu() always does __save_init_fpu() even if 
>>>> use_eager_fpu?
>>>> 
>>>> and note that in this case __thread_fpu_end() is wrong if 
>>>> use_eager_fpu, but fortunately the only possible caller of 
>>>> unlazy_fpu() is coredump. fpu_copy() checks use_eager_fpu().
>>>> 
>>>> - Is unlazy_fpu()->__save_init_fpu() safe wrt 
>>>> __kernel_fpu_begin() from irq?
> 
> It looks like it should be safe, as long as __save_init_fpu() knows
> that the task no longer has the FPU after __kernel_fpu_end(), so it
> does not try to save the kernel FPU state to the user's 
> task->thread.fpu.state->xstate
> 
> The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end() 
> needs to be kept from running during unlazy_fpu().
> 
> This means interrupted_kernel_fpu_idle and/or irq_fpu_usable need
> to check whether preemption is disabled, and lock out 
> __kernel_fpu_begin() when preemption is disabled.
> 
> It does not look like it currently does that...

... and that won't work, because preempt_disable() is a noop
without CONFIG_PREEMPT enabled. Sigh.

Not sure how to work around that, except by having
__Kernel_fpu_end() always restore the task FPU state, if the
task had the FPU when entering.

This suggests it may be advantageous to be liberal with
__thread_fpu_end() when we know user space will not be touching
the FPU again for a while, since that would reduce the work
done by __kernel_fpu_begin()/__kernel_fpu_end().

Does that make sense?

- -- 
All rights reversed
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