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Date: Sat, 14 Mar 2015 15:48:16 +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: <20150314144816.GA13029@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>
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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.