linux-kernel - Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush

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Date:	Sat, 14 Mar 2015 15:48:16 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in
	flush_thread()

On 03/13, Borislav Petkov wrote:
>
> On Fri, Mar 13, 2015 at 05:26:54PM +0100, Oleg Nesterov wrote:
> > > One example where drop_init_fpu() seems to make sense is
> > > __kernel_fpu_end(): kernel is done with FPU and current was using the
> > > FPU prior so let's restore it for the eagerfpu case.
> >
> > No, no, this is another case or I misunderstood you.
> >
> > __kernel_fpu_end() needs to restore FPU from current's fpu->state exactly
> > because current used FPU prior. And that state was saved by __save_init_fpu()
> > in __kernel_fpu_begin().
>
> That's exactly what I mean. See: "... kernel is done with FPU and current was
> using the FPU prior..."

Yes, but my point was that this is why we can _not_ use drop_init_fpu() in
__kernel_fpu_end().

Nevermind, look like I really misunderstood you.

Oleg.

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