linux-kernel - [PATCH 1/4] x86/fpu: document user_fpu

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Message-ID: <20150311173409.GC5032@redhat.com>
Date:	Wed, 11 Mar 2015 18:34:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dave Hansen <dave.hansen@...el.com>, Borislav Petkov <bp@...e.de>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: [PATCH 1/4] x86/fpu: document user_fpu_begin()

Currently user_fpu_begin() has a single caller and it is not clear that
why do we actually need it, and why we should not worry about preemption
right after preempt_enable().

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/include/asm/fpu-internal.h |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 4bec98f..c615ae9 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -464,7 +464,9 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
  * Need to be preemption-safe.
  *
  * NOTE! user_fpu_begin() must be used only immediately before restoring
- * it. This function does not do any save/restore on their own.
+ * it. This function does not do any save/restore on its own. In a lazy
+ * fpu mode this is just optimization to avoid a dna fault, the task can
+ * lose FPU right after preempt_enable().
  */
 static inline void user_fpu_begin(void)
 {
-- 
1.5.5.1


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