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Date: Fri, 13 Mar 2015 17:26:54 +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: <20150313162654.GA26453@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>
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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.