linux-kernel - Re: [PATCH] x86/fpu: always restore_xinit_state() when !use_eager

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Date:	Mon, 27 Apr 2015 17:00:16 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Bobby Powers <bobbypowers@...il.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Borislav Petkov <bp@...e.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/fpu: always restore_xinit_state() when
	!use_eager_cpu()

On 04/27, Dave Hansen wrote:
>
> On 04/26/2015 03:04 PM, Bobby Powers wrote:
>
> > The functional change was that flush_thread after f893959b only calls
> > restore_init_xstate when both !use_eager_fpu and !used_math are true.
> > drop_init_fpu (now fpu_reset_state) calls restore_init_xstate()
> > regardless of whether current used_math().
>
> This is really interesting.  We were seeing some issues where the xstate
> was not getting cleared across an exec, which seemed silly, but we just
> assumed it was something that had always been there.

This is because I am stupid.

Without this Bobby's fix flush_thread() simply does nothing if a user-space
task execs (if eagerfpu).

Oleg.

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