Re: [PATCH 1/4] x86/fpu: document user_fpu_begin()

[Borislav Petkov]

On Wed, Mar 11, 2015 at 06:34:09PM +0100, Oleg Nesterov wrote:
> 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@xxxxxxxxxx>
> ---
>  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().
>   */

I cleaned it up a bit more, if you don't mind:

---
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Date: Wed, 11 Mar 2015 18:34:09 +0100
Subject: [PATCH] x86/fpu: Document user_fpu_begin()

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

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Pekka Riikonen <priikone@xxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Suresh Siddha <sbsiddha@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20150311173409.GC5032@xxxxxxxxxx
Signed-off-by: 
---
 arch/x86/include/asm/fpu-internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 810f20fd4e4e..e8ee3da3b924 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -508,10 +508,12 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
 }
 
 /*
- * Need to be preemption-safe.
+ * Needs 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.
+ * the save state. It does not do any saving/restoring on its own. In
+ * lazy FPU mode, it is just an optimization to avoid a #NM exception,
+ * the task can lose the FPU right after preempt_enable().
  */
 static inline void user_fpu_begin(void)
 {
-- 

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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