arm64: add support for kernel mode NEON

Add <asm/neon.h> containing kernel_neon_begin/kernel_neon_end function
declarations and corresponding definitions in fpsimd.c

These are needed to wrap uses of NEON in kernel mode. The names are
identical to the ones used in arm/ so code using intrinsics or
vectorized by GCC can be shared between arm and arm64.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -96,6 +96,9 @@ config SWIOTLB
 config IOMMU_HELPER
 	def_bool SWIOTLB
 
+config KERNEL_MODE_NEON
+	def_bool y
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"

arch/arm64/include/asm/neon.h

+/*
+ * linux/arch/arm64/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.
+ */
+
+#define cpu_has_neon()		(1)
+
+void kernel_neon_begin(void);
+void kernel_neon_end(void);

arch/arm64/kernel/fpsimd.c

@@ -21,6 +21,7 @@
 #include <linux/init.h>
 #include <linux/sched.h>
 #include <linux/signal.h>
+#include <linux/hardirq.h>
 
 #include <asm/fpsimd.h>
 #include <asm/cputype.h>
@@ -83,6 +84,33 @@ void fpsimd_flush_thread(void)
 	fpsimd_load_state(&current->thread.fpsimd_state);
 }
 
+#ifdef CONFIG_KERNEL_MODE_NEON
+
+/*
+ * Kernel-side NEON support functions
+ */
+void kernel_neon_begin(void)
+{
+	/* Avoid using the NEON in interrupt context */
+	BUG_ON(in_interrupt());
+	preempt_disable();
+
+	if (current->mm)
+		fpsimd_save_state(&current->thread.fpsimd_state);
+}
+EXPORT_SYMBOL(kernel_neon_begin);
+
+void kernel_neon_end(void)
+{
+	if (current->mm)
+		fpsimd_load_state(&current->thread.fpsimd_state);
+
+	preempt_enable();
+}
+EXPORT_SYMBOL(kernel_neon_end);
+
+#endif /* CONFIG_KERNEL_MODE_NEON */
+
 /*
  * FP/SIMD support code initialisation.
  */