From bb840ef326ab3570d201e045e894710fd6db18f2 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Wed, 17 Sep 2025 15:06:35 -0400
Subject: kern/amd64: proc: Add initial context switch logic

This commit introduces new functions that will be useful for
having multiple processes run on the system.

- Add md_proc_yield()
- Add md_proc_kick()
- Add md_sched_switch()

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 src/sys/arch/amd64/cpu/trap.S   |  4 +-
 src/sys/arch/amd64/os/os_proc.c | 81 +++++++++++++++++++++++++++++++++++++++--
 src/sys/include/os/sched.h      |  8 ++++
 src/sys/include/sys/proc.h      | 14 +++++++
 4 files changed, 100 insertions(+), 7 deletions(-)

(limited to 'src/sys')

diff --git a/src/sys/arch/amd64/cpu/trap.S b/src/sys/arch/amd64/cpu/trap.S
index fba15ce..ef5c14f 100644
--- a/src/sys/arch/amd64/cpu/trap.S
+++ b/src/sys/arch/amd64/cpu/trap.S
@@ -104,8 +104,6 @@ TRAP_EXIT_EC(ss_fault)
 
     .globl lapic_tmr_isr
 INTR_ENTRY(lapic_tmr_isr)
-    // TODO
     mov %rsp, %rdi
-    cli
-    hlt
+    call md_sched_switch
 INTR_EXIT(lapic_tmr_isr)
diff --git a/src/sys/arch/amd64/os/os_proc.c b/src/sys/arch/amd64/os/os_proc.c
index 61ef51e..8c7b6f3 100644
--- a/src/sys/arch/amd64/os/os_proc.c
+++ b/src/sys/arch/amd64/os/os_proc.c
@@ -36,6 +36,7 @@
 #include <machine/pcb.h>
 #include <machine/gdt.h>
 #include <machine/frame.h>
+#include <machine/lapic.h>
 #include <string.h>
 
 /*
@@ -43,12 +44,16 @@
  * run. The catch is that we have never been in userland
  * from this context so we'll have to fake an IRET frame
  * to force the processor to have a CPL of 3.
- *
- * @tfp: Trapframe of context to enter
  */
-static inline void
-__proc_kick(struct trapframe *tfp)
+__dead void
+md_proc_kick(struct proc *procp)
 {
+    struct md_pcb *pcbp = &procp->pcb;
+    struct trapframe *tfp = &pcbp->tf;
+
+    mmu_write_vas(&pcbp->vas);
+    lapic_timer_oneshot_us(SCHED_QUANTUM);
+
     __ASMV(
         "sti\n"
         "mov %0, %%rax\n"
@@ -67,6 +72,8 @@ __proc_kick(struct trapframe *tfp)
           "m" (tfp->rflags),
           "r" (tfp->rip)
     );
+
+    __builtin_unreachable();
 }
 
 /*
@@ -121,6 +128,21 @@ md_proc_init(struct proc *procp, int flags)
     return 0;
 }
 
+/*
+ * Process idle loop
+ */
+__dead void
+md_proc_yield(void)
+{
+    /* Clear pending interrupts and oneshot */
+    lapic_eoi();
+    lapic_timer_oneshot_us(9000);
+
+    for (;;) {
+        __ASMV("sti; hlt");
+    }
+}
+
 /*
  * Set process instruction pointer
  */
@@ -139,3 +161,54 @@ md_set_ip(struct proc *procp, uintptr_t ip)
     tfp->rip = ip;
     return 0;
 }
+
+void
+md_sched_switch(struct trapframe *tf)
+{
+    struct proc *self, *proc = NULL;
+    struct md_pcb *pcbp;
+    struct pcore *core;
+    int error;
+
+    if ((core = this_core()) == NULL) {
+        printf("sched_switch: could not get core\n");
+        goto done;
+    }
+
+    /* Don't switch if no self */
+    if ((self = core->curproc) == NULL) {
+        md_proc_yield();
+    }
+
+    error = sched_enq(&core->scq, self);
+    if (error < 0) {
+        goto done;
+    }
+
+    /*
+     * Save the current trapframe to our process control
+     * block as we'll want it later when we're back.
+     */
+    pcbp = &self->pcb;
+    memcpy(&pcbp->tf, tf, sizeof(*tf));
+
+    /*
+     * Grab the next process. If we cannot find any, assume
+     * we are the only one and continue on...
+     */
+    error = sched_deq(&core->scq, &proc);
+    if (error < 0) {
+        goto done;
+    }
+
+    /* Load the next trapframe into the live one */
+    pcbp = &proc->pcb;
+    memcpy(tf, &pcbp->tf, sizeof(*tf));
+    core->curproc = proc;
+
+    /* Switch the address space and hope for the best */
+    mmu_write_vas(&pcbp->vas);
+done:
+    lapic_eoi();
+    lapic_timer_oneshot_us(SCHED_QUANTUM);
+}
diff --git a/src/sys/include/os/sched.h b/src/sys/include/os/sched.h
index b393395..1850366 100644
--- a/src/sys/include/os/sched.h
+++ b/src/sys/include/os/sched.h
@@ -35,6 +35,7 @@
 #include <sys/proc.h>
 #include <os/spinlock.h>
 
+#define SCHED_QUANTUM 10000
 #define SCHED_NQUEUES 4
 
 /*
@@ -49,6 +50,13 @@ struct sched_queue {
     size_t nproc;
 };
 
+/*
+ * Context switch to another process
+ *
+ * @tf: Current CPU context frame
+ */
+void md_sched_switch(struct trapframe *tf);
+
 /*
  * Enqueue a new process to a queue
  *
diff --git a/src/sys/include/sys/proc.h b/src/sys/include/sys/proc.h
index d09e459..59d47ec 100644
--- a/src/sys/include/sys/proc.h
+++ b/src/sys/include/sys/proc.h
@@ -31,6 +31,7 @@
 #define _SYS_PROC_H_
 
 #include <sys/types.h>
+#include <sys/cdefs.h>
 #include <sys/queue.h>
 #include <machine/pcb.h>    /* standard */
 
@@ -87,4 +88,17 @@ int md_proc_init(struct proc *procp, int flags);
  */
 int md_set_ip(struct proc *procp, uintptr_t ip);
 
+/*
+ * Put the current process into a halt loop
+ * until the next one runs.
+ */
+__dead void md_proc_yield(void);
+
+/*
+ * Kick a process into a user context
+ *
+ * @procp: Process pointer
+ */
+__dead void md_proc_kick(struct proc *procp);
+
 #endif  /* !_SYS_PROC_H_ */
-- 
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