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: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 136f263ed47b79d010cf4ca06e7b1e2a07e4fd73..d6068b435151eb02da433a65448bf28e0c9fe1c4 100644 (file)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -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"

diff --git a/arch/arm/include/asm/neon.h b/arch/arm/include/asm/neon.h
new file mode 100644 (file)
index 0000000..8f730fe
--- /dev/null
+++ b/arch/arm/include/asm/neon.h
@@ -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);

diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index 7620831a0c6651af16ab5911139458a66ff17899..52b8f40b1c73d48d206d497a6fadc381b5ddd888 100644 (file)
--- a/arch/arm/vfp/vfpmodule.c
+++ b/arch/arm/vfp/vfpmodule.c
@@ -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.
  */