[PATCH] asm-generic: simd: allow SIMD in process context with BH disabled

[Dave Martin]

On Wed, May 31, 2017 at 12:57:01PM +0000, Ard Biesheuvel wrote:
> 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 at zx2c4.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  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 at 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>
>  
>  /*
>   * may_use_simd - whether it is allowable at this time to issue SIMD
>   *                instructions or access the SIMD register file
>   *
>   * As architectures typically don't preserve the SIMD register file when
> - * taking an interrupt, !in_interrupt() should be a reasonable default.
> + * 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().


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

>   */
>  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?

Cheers
---Dave