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

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Message-ID: <20150313162654.GA26453@redhat.com>
Date:	Fri, 13 Mar 2015 17:26:54 +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 03:55:42PM +0100, Oleg Nesterov wrote:
> > But please look at drop_init_fpu(). If eagerfpu == F it calls drop_fpu() and
> > this is what we need. flush_thread() already has the "if (!use_eager_fpu())",
> > we can shift drop_fpu() there.
> >
> > Otherwise, if eagerfpu == T, drop_init_fpu() does restore_init_xstate() and
> > this just burns CPU. Until flush_thread user_has_fpu/used_math this state
> > restore_init_xstate() is pointless, this state will be lost after preemption.
>
> Yeah, I was wondering why that's there.
>
> 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().

Oleg.

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