abstract-machine(nemu): support context switch

abstract-machine/am/src/platform/nemu/trm.c

@@ -6,7 +6,7 @@ int main(const char *args);
 
 Area heap = RANGE(&_heap_start, PMEM_END);
 #ifndef MAINARGS
-#define MAINARGS "5"
+#define MAINARGS "i"
 #endif
 static const char mainargs[] = MAINARGS;
 

abstract-machine/am/src/riscv/nemu/cte.c

@@ -1,14 +1,21 @@
+#include "arch/riscv.h"
 #include <am.h>
-#include <riscv/riscv.h>
 #include <klib.h>
+#include <riscv/riscv.h>
+#include <stdint.h>
 
-static Context* (*user_handler)(Event, Context*) = NULL;
+static Context *(*user_handler)(Event, Context *) = NULL;
 
-Context* __am_irq_handle(Context *c) {
+Context *__am_irq_handle(Context *c) {
   if (user_handler) {
     Event ev = {0};
     switch (c->mcause) {
-      default: ev.event = EVENT_ERROR; break;
+    case CauseEnvironmentCallFromMMode:
+      ev.event = EVENT_YIELD;
+      break;
+    default:
+      ev.event = EVENT_ERROR;
+      break;
     }
 
     c = user_handler(ev, c);
@@ -20,7 +27,7 @@ Context* __am_irq_handle(Context *c) {
 
 extern void __am_asm_trap(void);
 
-bool cte_init(Context*(*handler)(Event, Context*)) {
+bool cte_init(Context *(*handler)(Event, Context *)) {
   // initialize exception entry
   asm volatile("csrw mtvec, %0" : : "r"(__am_asm_trap));
 
@@ -31,7 +38,10 @@ bool cte_init(Context*(*handler)(Event, Context*)) {
 }
 
 Context *kcontext(Area kstack, void (*entry)(void *), void *arg) {
-  return NULL;
+  Context *c = kstack.end - sizeof(Context);
+  c->mepc = (uintptr_t)entry;
+  c->gpr[10] = (uintptr_t)arg;
+  return c;
 }
 
 void yield() {
@@ -42,9 +52,6 @@ void yield() {
 #endif
 }
 
-bool ienabled() {
-  return false;
-}
+bool ienabled() { return false; }
 
-void iset(bool enable) {
-}
+void iset(bool enable) {}

abstract-machine/am/src/riscv/nemu/trap.S

@@ -60,6 +60,8 @@ __am_asm_trap:
   mv a0, sp
   jal __am_irq_handle
 
+  mv sp, a0
+
   LOAD t1, OFFSET_STATUS(sp)
   LOAD t2, OFFSET_EPC(sp)
   csrw mstatus, t1