From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752758AbbCOSTE (ORCPT <rfc822;w@1wt.eu>);
	Sun, 15 Mar 2015 14:19:04 -0400
Received: from mx1.redhat.com ([209.132.183.28]:47864 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751477AbbCOSTB (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Sun, 15 Mar 2015 14:19:01 -0400
Date: Sun, 15 Mar 2015 19:16:43 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in
	flush_thread()
Message-ID: <20150315181643.GA488@redhat.com>
References: <54F74F59.5070107@intel.com> <20150311173346.GB5032@redhat.com> <20150311173507.GF5032@redhat.com> <20150313105251.GD31998@pd.tnic> <20150313145542.GD21603@redhat.com> <20150313161958.GI31998@pd.tnic> <20150313162654.GA26453@redhat.com> <20150313192717.GJ31998@pd.tnic> <20150314144816.GA13029@redhat.com> <20150315173620.GA29134@pd.tnic>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150315173620.GA29134@pd.tnic>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 03/15, Borislav Petkov wrote:
>
> On Sat, Mar 14, 2015 at 03:48:16PM +0100, Oleg Nesterov wrote:
> > > >
> > > > __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().
>
> Hmm, now I'm confused.

Me too...

> void __kernel_fpu_end():
>
> ...
>
>         if (__thread_has_fpu(me)) {
>                 if (WARN_ON(restore_fpu_checking(me)))
>
> restore_fpu_checking(current) does try to restore fpu->state and it does
> drop_init_fpu() only if it failed.
>
> Ok, now you tell me what I'm missing :)

Of course, drop_init_fpu() is fine if restore_fpu_checking() fails.

Did you mean this from the very beginning? In this case I agree of course.

Because I misinterpreted your initial comment:

	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.
	
as if you suggest to use it _instead_ of restore_fpu_checking().

Oleg.