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riscv: implement arch_switch()

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The move to arch_switch() is a prerequisite for SMP support.

Make it optimal without the need for an ECALL roundtrip on every
context switch. Performance numbers from tests/benchmarks/sched:

Before:
unpend  107 ready  102 switch  188 pend  218 tot  615 (avg  615)

After:
unpend  107 ready  102 switch  170 pend  217 tot  596 (avg  595)

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
This commit is contained in:
Nicolas Pitre 2022-03-07 17:01:36 -05:00 • committed by Anas Nashif
commit ce8dabfe9e
12 changed files with 197 additions and 234 deletions
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11
arch/riscv/core/thread.c
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@ -23,6 +23,7 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
char *stack_ptr, k_thread_entry_t entry,
void *p1, void *p2, void *p3)
{
extern void z_riscv_thread_start(void);
struct __esf *stack_init;
#ifdef CONFIG_RISCV_SOC_CONTEXT_SAVE
@ -53,8 +54,8 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
* and restored prior to returning from the interrupt/exception.
* This shall allow to handle nested interrupts.
*
* Given that context switching is performed via a system call exception
* within the RISCV architecture implementation, initially set:
* Given that thread startup happens through the exception exit
* path, initially set:
* 1) MSTATUS to MSTATUS_DEF_RESTORE in the thread stack to enable
* interrupts when the newly created thread will be scheduled;
* 2) MEPC to the address of the z_thread_entry in the thread
@ -121,6 +122,12 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
#endif
thread->callee_saved.sp = (ulong_t)stack_init;
/* where to go when returning from z_riscv_switch() */
thread->callee_saved.ra = (ulong_t)z_riscv_thread_start;
/* our switch handle is the thread pointer itself */
thread->switch_handle = thread;
}
#if defined(CONFIG_FPU) && defined(CONFIG_FPU_SHARING)