Commit 73c132c1 authored by Ard Biesheuvel's avatar Ard Biesheuvel
Browse files

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.

Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: default avatarNicolas Pitre <nico@linaro.org>
parent ab3da156

arch/arm/Kconfig

+7 −0
Original line number Diff line number Diff line
@@ -2197,6 +2197,13 @@ 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"

arch/arm/include/asm/neon.h

0 → 100644
+36 −0
Original line number Diff line number Diff line 
/*

 * 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);

arch/arm/vfp/vfpmodule.c

+47 −0
Original line number Diff line number Diff line
@@ -20,6 +20,7 @@ 
#include <linux/init.h>

#include <linux/uaccess.h>

#include <linux/user.h>

#include <linux/export.h>



#include <asm/cp15.h>

#include <asm/cputype.h>
@@ -668,6 +669,52 @@ void vfp_kmode_exception(void) 
		pr_crit("BUG: FP instruction issued in kernel mode with FP unit disabled\n");

}



#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 support code initialisation.

 */

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