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ARM: add support for kernel mode NEON 

In order to safely support the use of NEON instructions in
kernel mode, some precautions need to be taken:
- the userland context that may be present in the registers (even
  if the NEON/VFP is currently disabled) must be stored under the
  correct task (which may not be 'current' in the UP case),
- to avoid having to keep track of additional vfpstates for the
  kernel side, disallow the use of NEON in interrupt context
  and run with preemption disabled,
- after use, re-enable preemption and re-enable the lazy restore
  machinery by disabling the NEON/VFP unit.

This patch adds the functions kernel_neon_begin() and
kernel_neon_end() which take care of the above. It also adds
the Kconfig symbol KERNEL_MODE_NEON to enable it.

Change-Id: I595345a0eabcc55685582ed3df1c2968bf49e037
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Git-commit: 3ad7b1bb81d15ab510cce9dc9df7f1a18185ee36
Git-repo: https://android.googlesource.com/kernel/common/
Signed-off-by: Samir Mehta <samirn@codeaurora.org>
This commit is contained in:
Ard Biesheuvel 2013-05-16 11:41:48 +02:00 committed by Zhao Wei Liew
parent 17d5fd2500
commit 0c8bac0bf0
3 changed files with 90 additions and 0 deletions
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7
arch/arm/Kconfig
View File

@ -2369,6 +2369,13 @@ config NEON
Say Y to include support code for NEON, the ARMv7 Advanced SIMD
Extension.
config KERNEL_MODE_NEON
bool "Support for NEON in kernel mode"
default n
depends on NEON
help
Say Y to include support for NEON in kernel mode.
endmenu
menu "Userspace binary formats"

36
arch/arm/include/asm/neon.h Normal file
View File

@ -0,0 +1,36 @@
/*
* linux/arch/arm/include/asm/neon.h
*
* Copyright (C) 2013 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 <asm/hwcap.h>
#define cpu_has_neon() (!!(elf_hwcap & HWCAP_NEON))
#ifdef __ARM_NEON__
/*
* If you are affected by the BUILD_BUG below, it probably means that you are
* using NEON code /and/ calling the kernel_neon_begin() function from the same
* compilation unit. To prevent issues that may arise from GCC reordering or
* generating(1) NEON instructions outside of these begin/end functions, the
* only supported way of using NEON code in the kernel is by isolating it in a
* separate compilation unit, and calling it from another unit from inside a
* kernel_neon_begin/kernel_neon_end pair.
*
* (1) Current GCC (4.7) might generate NEON instructions at O3 level if
* -mpfu=neon is set.
*/
#define kernel_neon_begin() \
BUILD_BUG_ON_MSG(1, "kernel_neon_begin() called from NEON code")
#else
void kernel_neon_begin(void);
#endif
void kernel_neon_end(void);

47
arch/arm/vfp/vfpmodule.c
View File

@ -22,6 +22,7 @@
#include <linux/user.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/export.h>
#include <asm/cp15.h>
#include <asm/cputype.h>
@ -636,6 +637,52 @@ int vfp_restore_user_hwstate(struct user_vfp __user *ufp,
return err ? -EFAULT : 0;
}
#ifdef CONFIG_KERNEL_MODE_NEON
/*
* Kernel-side NEON support functions
*/
void kernel_neon_begin(void)
{
struct thread_info *thread = current_thread_info();
unsigned int cpu;
u32 fpexc;
/*
* Kernel mode NEON is only allowed outside of interrupt context
* with preemption disabled. This will make sure that the kernel
* mode NEON register contents never need to be preserved.
*/
BUG_ON(in_interrupt());
cpu = get_cpu();
fpexc = fmrx(FPEXC) | FPEXC_EN;
fmxr(FPEXC, fpexc);
/*
* Save the userland NEON/VFP state. Under UP,
* the owner could be a task other than 'current'
*/
if (vfp_state_in_hw(cpu, thread))
vfp_save_state(&thread->vfpstate, fpexc);
#ifndef CONFIG_SMP
else if (vfp_current_hw_state[cpu] != NULL)
vfp_save_state(vfp_current_hw_state[cpu], fpexc);
#endif
vfp_current_hw_state[cpu] = NULL;
}
EXPORT_SYMBOL(kernel_neon_begin);
void kernel_neon_end(void)
{
/* Disable the NEON/VFP unit. */
fmxr(FPEXC, fmrx(FPEXC) & ~FPEXC_EN);
put_cpu();
}
EXPORT_SYMBOL(kernel_neon_end);
#endif /* CONFIG_KERNEL_MODE_NEON */
/*
* VFP hardware can lose all context when a CPU goes offline.
* As we will be running in SMP mode with CPU hotplug, we will save the