kernel: Add alternative _arch_switch context switch primitive

The existing __swap() mechanism is too high level for some
applications because of its scheduler-awareness.  This introduces a
new _arch_switch() mechanism, which is a simpler primitive that looks
like:

    void _arch_switch(void *handle, void **old_handle_out);

The new thread handle (typically just a stack pointer) is specified
explicitly instead of being picked up from the scheduler by
per-architecture code, and on return the "old" thread handle that got
switched out is returned through the pointer.

The new primitive (currently available only on xtensa) is selected
when CONFIG_USE_SWITCH is "y".  A new C _Swap() implementation based
on this primitive is then added which operates compatibly.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

arch/xtensa/include/kernel_arch_data.h

@@ -54,6 +54,10 @@ typedef struct _kernel_arch _kernel_arch_t;
 
 #endif /*! _ASMLANGUAGE && ! __ASSEMBLER__ */
 
+#ifdef CONFIG_USE_SWITCH
+void xtensa_switch(void *switch_to, void **switched_from);
+#define _arch_switch xtensa_switch
+#endif
 
 /* stacks */
 #define STACK_ROUND_UP(x) ROUND_UP(x, STACK_ALIGN_SIZE)

include/kernel.h

@@ -469,6 +469,18 @@ struct k_thread {
 	k_thread_stack_t *stack_obj;
 #endif /* CONFIG_USERSPACE */
 
+#if defined(CONFIG_USE_SWITCH)
+	/* When using __switch() a few previously arch-specific items
+	 * become part of the core OS
+	 */
+
+	/* _Swap() return value */
+	int swap_retval;
+
+	/* Context handle returned via _arch_switch() */
+	void *switch_handle;
+#endif
+
 	/* arch-specifics: must always be at the end */
 	struct _thread_arch arch;
 };

kernel/Kconfig

@@ -549,6 +549,18 @@ config MAX_DOMAIN_PARTITIONS
 	help
 	  Configure the maximum number of partitions per memory domain.
 
+config USE_SWITCH
+	bool
+	prompt "Use new-style _arch_switch instead of __swap"
+	default n
+	help
+	The _arch_switch() API is a lower level context switching
+	primitive than the original __swap mechanism.  It is required
+	for an SMP-aware scheduler, or if the architecture does not
+	provide __swap.  In uniprocess situations where the
+	architecture provides both, _arch_switch incurs more somewhat
+	overhead and may be slower.
+
 source "kernel/Kconfig.event_logger"
 
 source "kernel/Kconfig.power_mgmt"

kernel/include/kernel_internal.h

@@ -16,6 +16,7 @@
 
 #include <kernel.h>
 #include <kernel_structs.h>
+#include <ksched.h>
 
 #ifndef _ASMLANGUAGE
 
@@ -51,30 +52,61 @@ extern void _setup_new_thread(struct k_thread *new_thread,
 			      void *p1, void *p2, void *p3,
 			      int prio, u32_t options);
 
-/* context switching and scheduling-related routines */
-
-extern unsigned int __swap(unsigned int key);
-
 #ifdef CONFIG_TIMESLICING
 extern void _update_time_slice_before_swap(void);
+#else
+#define _update_time_slice_before_swap() /**/
 #endif
 
 #ifdef CONFIG_STACK_SENTINEL
 extern void _check_stack_sentinel(void);
+#else
+#define _check_stack_sentinel() /**/
 #endif
 
+/* context switching and scheduling-related routines */
+#ifdef CONFIG_USE_SWITCH
+
+/* New style context switching.  _arch_switch() is a lower level
+ * primitive that doesn't know about the scheduler or return value.
+ * Needed for SMP, where the scheduler requires spinlocking that we
+ * don't want to have to do in per-architecture assembly.
+ */
+static inline unsigned int _Swap(unsigned int key)
+{
+	struct k_thread *new_thread, *old_thread;
+
+	old_thread = _kernel.current;
+
+	_check_stack_sentinel();
+	_update_time_slice_before_swap();
+
+	new_thread = _get_next_ready_thread();
+
+	old_thread->swap_retval = -EAGAIN;
+
+	_kernel.current = new_thread;
+	_arch_switch(new_thread->switch_handle,
+		     &old_thread->switch_handle);
+
+	irq_unlock(key);
+
+	return _kernel.current->swap_retval;
+}
+
+#else /* !CONFIG_USE_SWITCH */
+
+extern unsigned int __swap(unsigned int key);
+
 static inline unsigned int _Swap(unsigned int key)
 {
-
-#ifdef CONFIG_STACK_SENTINEL
 	_check_stack_sentinel();
-#endif
-#ifdef CONFIG_TIMESLICING
 	_update_time_slice_before_swap();
-#endif
 
 	return __swap(key);
 }
+#endif
+
 
 #ifdef CONFIG_USERSPACE
 /**