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

[Dave Martin]

On Wed, May 31, 2017 at 03:52:48PM +0000, Ard Biesheuvel wrote:
> On 31 May 2017 at 15:32, Dave Martin <Dave.Martin at arm.com> wrote:
> > 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>

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

[...]

> >> + * 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.
> 
> >
> > 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?
> >
> 
> I agree. I will add that as well.

OK, cheers
---Dave