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>

1 files changed, 47 insertions, 0 deletions

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.
  */