Added function to iret from exit() syscall

etc/amd64/gdbrc

@@ -0,0 +1,3 @@
+target remote localhost:1234
+layout regs
+symbol-file build/sys/amd64/kernel.elf

sys/amd64/sys/sched.c

@@ -201,51 +201,39 @@ void sched_init(void) {
     sched_add_to(0, &t_init);
 }
 
+void sched_remove_from(int cpu, struct thread *thr) {
+    struct thread *prev = thr->prev;
+    struct thread *next = thr->next;
+    kdebug("cpu%d: removing thread %d\n", cpu, thr->pid);
+
+    _assert(prev);
+    prev->next = next;
+
+    if (next) {
+        next->prev = prev;
+    } else {
+        sched_queue_tails[cpu] = prev;
+    }
+
+    --sched_queue_sizes[cpu];
+
+    thr->next = NULL;
+    thr->prev = NULL;
+
+    // TODO: make other cpus schedule a new thread if they're running a stopped one
+}
+
+void sched_remove(struct thread *thr) {
+    kdebug("Remove %p\n", thr);
+    sched_remove_from(thr->cpu, thr);
+}
+
 int sched(void) {
     struct thread *from = get_cpu()->thread;
     struct thread *to;
     int cpu = get_cpu()->processor_id;
     uintptr_t flags;
 
-    // Cleanup stopped tasks
-    // TODO: this won't be needed if I call sched_remove directly
-    to = sched_queue_heads[cpu];
-    while (to) {
-        if (to->flags & THREAD_STOPPED) {
-            // So we don't remove [idle]
-            _assert(to != sched_queue_heads[cpu]);
-            kdebug("(cpu%d) Removing %u from queue\n", cpu, to->pid);
-
-            if (to->pid == 1) {
-                panic("init process was killed\n");
-            }
-
-            struct thread *prev, *next;
-            prev = to->prev;
-            next = to->next;
-
-            _assert(prev);
-
-            prev->next = next;
-            if (next) {
-                next->prev = prev;
-            } else {
-                sched_queue_tails[cpu] = prev;
-            }
-
-            // TODO: free the task?
-            if (to == from) {
-                // Removing current thread
-                from = NULL;
-            }
-
-            if (!next) {
-                break;
-            }
-        }
-        to = to->next;
-    }
-
     if (from && from->next) {
         to = from->next;
     } else {

sys/amd64/syscall.c

@@ -14,7 +14,7 @@ static void sys_close(int fd);
 static int sys_stat(const char *filename, struct stat *st);
 static void sys_exit(int status);
 
-uint8_t irq1_key = 0;
+__attribute__((noreturn)) void amd64_syscall_yield_stopped(void);
 
 intptr_t amd64_syscall(uintptr_t rdi, uintptr_t rsi, uintptr_t rdx, uintptr_t rcx, uintptr_t r10, uintptr_t rax) {
     switch (rax) {
@@ -32,12 +32,8 @@ intptr_t amd64_syscall(uintptr_t rdi, uintptr_t rsi, uintptr_t rdx, uintptr_t rc
 
     case SYSCALL_NR_EXIT:
         sys_exit((int) rdi);
-        // TODO: call sched_remove and reschedule a new task so that
-        // this one does not return
-        asm volatile ("sti; hlt");
-        panic("This should not happen\n");
-        // WTF?
-        return -1;
+        amd64_syscall_yield_stopped();
+
     default:
         kdebug("unknown syscall: %u\n", rax);
         return -1;

sys/amd64/syscall_s.S

@@ -1,7 +1,9 @@
 #include "sys/amd64/asm/asm_cpu.h"
+#include "sys/amd64/asm/asm_irq.h"
 
 .section .text
 .global amd64_syscall_init
+.global amd64_syscall_yield_stopped
 
 #define MSR_STAR        0xC0000081
 #define MSR_LSTAR       0xC0000082
@@ -104,5 +106,41 @@ amd64_syscall_entry:
     swapgs
     sysretq
 
-_fmt0:
-    .string "rsp = %p\n"
+// Schedule control to a new task instead of stopped one
+amd64_syscall_yield_stopped:
+    cli
+    // No stack frame - the kernel stack is going to be discarded anyways
+    movq get_cpu(0x08), %rbx
+
+    call sched
+    test %rax, %rax
+    jz 1f
+
+    // sched() failed
+    leaq _yield_failed(%rip), %rdi
+    xorq %rax, %rax
+    call panicf
+
+1:
+    // Remove old thread from queues
+    movq %rbx, %rdi
+    call sched_remove
+
+    // Load a new context
+    // rsi = thread structure pointer
+    movq get_cpu(0x08), %rsi
+    movq 0(%rsi), %rsp
+
+    // Write TSS entry
+    movq get_cpu(0x18), %rdi
+    movq %rsp, %rax
+    addq $(25 * 8), %rax
+    movq %rax, 4(%rdi)
+1:
+    // Assume we're now on TASK2's stack
+    irq_pop_ctx
+
+    iretq
+
+_yield_failed:
+    .string "sched() returned -1 on amd64_syscall_yield\n"