On 31 May 2017 at 16:13, Dave Martin [off-list ref] wrote:

On Wed, May 31, 2017 at 03:52:48PM +0000, Ard Biesheuvel wrote:

quoted

On 31 May 2017 at 15:32, Dave Martin [off-list ref] wrote:

quoted

On Wed, May 31, 2017 at 12:57:01PM +0000, Ard Biesheuvel wrote:

quoted

asm-generic supplies a header asm/simd.h which exports a single function
may_use_simd(), which conveys whether the current context allows the SIMD
register file or instructions to be used.

This header is included by crypto code shared between x86 and ARM/arm64,
and which offloads SIMD processing to process context if required. The
generic asm/simd.h is shared between ARM and arm64 at the moment, while
x86 has its own implementation.

On arm64, we currently mostly ignore may_use_simd(), because arm64 allows
kernel mode NEON in any context. However, this is due to change shortly
when support for SVE is merged, at which point we will introduce an arm64
specific implementation of asm/simd.h as well.

That leaves ARM, which only allows kernel mode NEON in process context,
which makes the current generic implementation of may_use_simd() seem
appropriate. However, given that in_interrupt() will return true when
running in process context with bottom halves disabled, we may end up
falling back to less optimized code unnecessarily, given that kernel
mode NEON is perfectly usable in that case.

So redefine may_use_simd() to disallow SIMD only when running in hardirq
or softirq context.

While we're at it, add some missing header file decorations such as
a license header and include guards.

Reported-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Signed-off-by: Ard Biesheuvel <redacted>
---
 include/asm-generic/simd.h | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/simd.h b/include/asm-generic/simd.h
index f57eb7b5c23b..a3e5ebe6b2b2 100644
--- a/include/asm-generic/simd.h
+++ b/include/asm-generic/simd.h

@@ -1,14 +1,31 @@
+/*
+ * Copyright (C) 2013 - 2017 Linaro Ltd. <ard.biesheuvel@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published
+ * by the Free Software Foundation.
+ */

-#include <linux/hardirq.h>
+#ifndef __ASM_SIMD_H
+#define __ASM_SIMD_H
+
+#include <linux/types.h>
+#include <linux/preempt.h>

Forgot to mention, should <linux/compiler.h> be included for
__must_check?

Yes.

quoted

quoted

quoted

+ * taking an interrupt, it is reasonable to define the default behavior
+ * of 'may_use_simd()' to be 'SIMD is only allowed when not handling an
+ * IRQ or softIRQ'. Since 'in_interrupt()' will also return true when
+ * running in process context with bottom halves disabled, we have to
+ * spell out that condition as shown.

Minor nit: do we need the comment about in_interrupt() here?

It makes more sense to explain the change in the commit message (which
you do) than to explain in-line the behaviour of a function that the
code doesn't use.

<linux/preempt.h> already hints at the caveats of in_interrupt().

Fair enough. I tend to err on the verbose side when it comes to
comments, but this could indeed be omitted.

quoted

For this comment block, it may be more helpful to note that SIMD is
permitted in task context even if bottom halves are enabled.

quoted

  */
 static __must_check inline bool may_use_simd(void)
 {
-     return !in_interrupt();
+     return !in_irq() && !in_serving_softirq();

Previously, in_nmi() implied !may_use_simd().

Now, may_use_simd() can return true if in_nmi().

Code in NMI context probably shouldn't be touching this interface at
all, but we may want to close this hole by adding && !in_nmi()
explicitly.  I did that in my kernel-mode-neon simplification series,
but couldn't decide whether it was superfluous.

Any thoughts?

I agree. I will add that as well.

OK, cheers
---Dave