Re: [PATCH] x86/fpu: always restore_xinit_state() when !use_eager_cpu()

From: Oleg Nesterov <oleg@redhat.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Bobby Powers <bobbypowers@gmail.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	Borislav Petkov <bp@suse.de>, Ingo Molnar <mingo@kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Pekka Riikonen <priikone@iki.fi>,
	Quentin Casasnovas <quentin.casasnovas@oracle.com>,
	Rik van Riel <riel@redhat.com>,
	Suresh Siddha <sbsiddha@gmail.com>,
	Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/fpu: always restore_xinit_state() when !use_eager_cpu()
Date: Mon, 27 Apr 2015 17:00:16 +0200	[thread overview]
Message-ID: <20150427150016.GA26871@redhat.com> (raw)
In-Reply-To: <553E4BB2.2040603@intel.com>

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.