sparc: Add SPARC_USE_SAFE_FP_SUPPORT

The SPARC ABI is a bit special with respect to the floating point context.
The complete floating point context is volatile.  Thus from an ABI point
of view nothing needs to be saved and restored during a context switch.
Instead the floating point context must be saved and restored during
interrupt processing.  Historically the deferred floating point switch is
used for SPARC and the complete floating point context is saved and
restored during a context switch to the new floating point unit owner.
This is a bit dangerous since post-switch actions (e.g. signal handlers)
and context switch extensions may silently corrupt the floating point
context.  The floating point unit is disabled for interrupt handlers.
Thus in case an interrupt handler uses the floating point unit then this
will result in a trap.

On SMP configurations the deferred floating point switch is not
supported in principle.  So use here a safe floating point support.  Safe
means that the volatile floating point context is saved and restored
around a thread dispatch issued during interrupt processing.  Thus
post-switch actions and context switch extensions may safely use the
floating point unit.

Update #2270.

3 files changed, 29 insertions, 4 deletions

diff --git a/cpukit/score/cpu/sparc/cpu.c b/cpukit/score/cpu/sparc/cpu.c
index bab0040058..569b6f8ec8 100644
--- a/cpukit/score/cpu/sparc/cpu.c
+++ b/cpukit/score/cpu/sparc/cpu.c
@@ -131,7 +131,7 @@ RTEMS_STATIC_ASSERT(
 
 void _CPU_Initialize(void)
 {
-#if (SPARC_HAS_FPU == 1)
+#if (SPARC_HAS_FPU == 1) && !defined(SPARC_USE_SAFE_FP_SUPPORT)
   Context_Control_fp *pointer;
   uint32_t            psr;
 
diff --git a/cpukit/score/cpu/sparc/cpu_asm.S b/cpukit/score/cpu/sparc/cpu_asm.S
index 3fa0532c86..4e683dae0d 100644
--- a/cpukit/score/cpu/sparc/cpu_asm.S
+++ b/cpukit/score/cpu/sparc/cpu_asm.S
@@ -26,7 +26,7 @@
 #include <rtems/asm.h>
 #include <rtems/system.h>
 
-#if (SPARC_HAS_FPU == 1)
+#if (SPARC_HAS_FPU == 1) && !defined(SPARC_USE_SAFE_FP_SUPPORT)
 
 /*
  *  void _CPU_Context_save_fp(
diff --git a/cpukit/score/cpu/sparc/rtems/score/cpu.h b/cpukit/score/cpu/sparc/rtems/score/cpu.h
index 02891b0438..d35846521e 100644
--- a/cpukit/score/cpu/sparc/rtems/score/cpu.h
+++ b/cpukit/score/cpu/sparc/rtems/score/cpu.h
@@ -28,6 +28,31 @@ extern "C" {
 
 /* conditional compilation parameters */
 
+#if defined(RTEMS_SMP)
+  /*
+   * The SPARC ABI is a bit special with respect to the floating point context.
+   * The complete floating point context is volatile.  Thus from an ABI point
+   * of view nothing needs to be saved and restored during a context switch.
+   * Instead the floating point context must be saved and restored during
+   * interrupt processing.  Historically the deferred floating point switch is
+   * used for SPARC and the complete floating point context is saved and
+   * restored during a context switch to the new floating point unit owner.
+   * This is a bit dangerous since post-switch actions (e.g. signal handlers)
+   * and context switch extensions may silently corrupt the floating point
+   * context.  The floating point unit is disabled for interrupt handlers.
+   * Thus in case an interrupt handler uses the floating point unit then this
+   * will result in a trap.
+   *
+   * On SMP configurations the deferred floating point switch is not
+   * supported in principle.  So use here a safe floating point support.  Safe
+   * means that the volatile floating point context is saved and restored
+   * around a thread dispatch issued during interrupt processing.  Thus
+   * post-switch actions and context switch extensions may safely use the
+   * floating point unit.
+   */
+  #define SPARC_USE_SAFE_FP_SUPPORT
+#endif
+
 /**
  * Should the calls to _Thread_Enable_dispatch be inlined?
  *
@@ -102,7 +127,7 @@ extern "C" {
  *
  * This is set based upon the multilib settings.
  */
-#if ( SPARC_HAS_FPU == 1 )
+#if ( SPARC_HAS_FPU == 1 ) && !defined(SPARC_USE_SAFE_FP_SUPPORT)
   #define CPU_HARDWARE_FP     TRUE
 #else
   #define CPU_HARDWARE_FP     FALSE
@@ -152,7 +177,7 @@ extern "C" {
  * On the SPARC, we can disable the FPU for integer only tasks so
  * it is safe to defer floating point context switches.
  */
-#if defined(RTEMS_SMP)
+#if defined(SPARC_USE_SAFE_FP_SUPPORT)
   #define CPU_USE_DEFERRED_FP_SWITCH FALSE
 #else
   #define CPU_USE_DEFERRED_FP_SWITCH TRUE