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ARM: add support for kernel mode NEON

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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>
This commit is contained in:
Ard Biesheuvel
2013-05-16 11:41:48 +02:00
parent ab3da15643
commit 73c132c15d
3 changed files with 90 additions and 0 deletions
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47
arch/arm/vfp/vfpmodule.c
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@@ -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.
*/