arch/arm64 :Cyclic Test fix in ARM64 fpsimd

Please follow the discussion in kernel mailing list about this patch for clear update.

We are able to achieve very good latency with this below fix. Lot of discussion going in linux kernel mailing list to make it better.

From e6a5fce9b3b55f48656240036a9354a0997c2907 Mon Sep 17 00:00:00 2001
From: Ayyappa Ch <ayyappa.chandolu@amd.com>
Date: Tue, 18 Apr 2015 11:53:00 +0530
Subject: [PATCH ] floating point realtime fix

---
 arch/arm64/kernel/fpsimd.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 2438497..3dca156 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -166,10 +166,10 @@ void fpsimd_flush_thread(void)
  */
 void fpsimd_preserve_current_state(void)
 {
- preempt_disable();
+ migrate_disable();
  if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
  fpsimd_save_state(&current->thread.fpsimd_state);
- preempt_enable();
+ migrate_enable();
 }

 /*
@@ -179,7 +179,7 @@ void fpsimd_preserve_current_state(void)
  */
 void fpsimd_restore_current_state(void)
 {
- preempt_disable();
+ migrate_disable();
  if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
  struct fpsimd_state *st = &current->thread.fpsimd_state;

@@ -187,7 +187,7 @@ void fpsimd_restore_current_state(void)
  this_cpu_write(fpsimd_last_state, st);
  st->cpu = smp_processor_id();
  }
- preempt_enable();
+ migrate_enable();
 }

 /*
@@ -197,7 +197,7 @@ void fpsimd_restore_current_state(void)
  */
 void fpsimd_update_current_state(struct fpsimd_state *state)
 {
- preempt_disable();
+ migrate_disable();
  fpsimd_load_state(state);
  if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
  struct fpsimd_state *st = &current->thread.fpsimd_state;
@@ -205,7 +205,7 @@ void fpsimd_update_current_state(struct fpsimd_state *state)
  this_cpu_write(fpsimd_last_state, st);
  st->cpu = smp_processor_id();
  }
- preempt_enable();
+ migrate_enable();
 }

 /*
@@ -239,7 +239,7 @@ void kernel_neon_begin_partial(u32 num_regs)
  * that there is no longer userland FPSIMD state in the
  * registers.
  */
- preempt_disable();
+ migrate_disable();
  if (current->mm &&
     !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE))
  fpsimd_save_state(&current->thread.fpsimd_state);
@@ -255,7 +255,7 @@ void kernel_neon_end(void)
  in_irq() ? &hardirq_fpsimdstate : &softirq_fpsimdstate);
  fpsimd_load_partial_state(s);
  } else {
- preempt_enable();
+ migrate_enable();
  }
 }
 EXPORT_SYMBOL(kernel_neon_end);
--
1.9.1

It seems  the kernel won't save the state of the simd registers when it is preempted so if another task runs on the same CPU and also uses simd, it clobbers the registers of the first task, and migrate_disable() does not
prevent that. So , with the above patch there is a possibility that simd instruction will get corrupted.

Keep watching the kernel mailing list about this issue for the final fix to improve latency issue with fpsimd .