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Date: Sun, 15 Mar 2015 21:04:36 +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>,
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        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: <20150315200436.GA30079@redhat.com>
References: <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> <20150315181643.GA488@redhat.com> <20150315185048.GB29134@pd.tnic>
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On 03/15, Borislav Petkov wrote:
>
> On Sun, Mar 15, 2015 at 07:16:43PM +0100, Oleg Nesterov wrote:
>
> Anyway, we're on the same page and that was a good exercise :-)

Yes, finally ;)

> +/*
> + * In addition to "forgetting" FPU state for @tsk, we restore the
> + * default FPU state in the eager case. Note, this is not needed in the
> + * non-eager case because there we will set CR0.TS and fault and setup
> + * an FPU state lazily.
> + *
> + * We restore the default FPU state in the eager case here as a means of
> + * addressing the failure of restoring the FPU state which @tsk points
> + * to and we still need some state to use so we use the default, clean
> + * one.
> + */
>  static inline void drop_init_fpu(struct task_struct *tsk)
>  {
>  	if (!use_eager_fpu())

But please note that it is not only used after the failure.
See handle_signal() and the first drop_init_fpu() in __restore_xstate_sig().

I think its name is confusing a bit...

Oleg.