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authorArd Biesheuvel <ard.biesheuvel@linaro.org>2013-05-16 11:41:48 +0200
committerArd Biesheuvel <ard.biesheuvel@linaro.org>2013-07-08 22:08:51 +0100
commit73c132c15da504789b924871e2491479a18e4f6a (patch)
treee5aab4cb0d9263419677a4aa9aea392ffad76053
parentab3da15643469ab2d206dee3d9cfa4194ba77f25 (diff)
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>
-rw-r--r--arch/arm/Kconfig7
-rw-r--r--arch/arm/include/asm/neon.h36
-rw-r--r--arch/arm/vfp/vfpmodule.c47
3 files changed, 90 insertions, 0 deletions
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 136f263ed47b..d6068b435151 100644
--- 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
index 000000000000..8f730fe70093
--- /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 7620831a0c66..52b8f40b1c73 100644
--- 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.
*/