From e3fba3a2fc865a5b4d6fa622fd383b6d612e8c85 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Tue, 16 Sep 2025 20:53:43 -0400
Subject: kern: proc: Add initialization code for ring 3

This commit introduces several new functions for switching the processor
into a ring three context along with its task's respective process
descriptors to be implemented correctly.

- Add md_set_ip() to update proc instruction pointer
- Add stack definitions
- Add stack mapping logic
...

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 src/sys/arch/amd64/os/os_proc.c | 88 +++++++++++++++++++++++++++++++++++++++++
 src/sys/include/sys/proc.h      | 18 +++++++++
 2 files changed, 106 insertions(+)

(limited to 'src/sys')

diff --git a/src/sys/arch/amd64/os/os_proc.c b/src/sys/arch/amd64/os/os_proc.c
index 8fad7b6..7a65b34 100644
--- a/src/sys/arch/amd64/os/os_proc.c
+++ b/src/sys/arch/amd64/os/os_proc.c
@@ -30,8 +30,44 @@
 #include <sys/errno.h>
 #include <sys/proc.h>
 #include <sys/syslog.h>
+#include <sys/cdefs.h>
 #include <vm/mmu.h>
+#include <vm/map.h>
 #include <machine/pcb.h>
+#include <machine/gdt.h>
+#include <machine/frame.h>
+#include <string.h>
+
+/*
+ * Put the current process into userland for its first
+ * 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)
+{
+    __ASMV(
+        "sti\n"
+        "mov %0, %%rax\n"
+        "push %1\n"
+        "push %2\n"
+        "push %3\n"
+        "push %%rax\n"
+        "push %4\n"
+        "lfence\n"
+        "swapgs\n"
+        "iretq"
+        :
+        : "r" (tfp->cs),
+          "i" (USER_DS | 3),
+          "r" (tfp->rsp),
+          "m" (tfp->rflags),
+          "r" (tfp->rip)
+    );
+}
 
 /*
  * MD proc init code
@@ -40,6 +76,9 @@ int
 md_proc_init(struct proc *procp, int flags)
 {
     struct md_pcb *pcbp;
+    struct trapframe *tfp;
+    struct mmu_map spec;
+    uint8_t cs, ds;
     int error;
 
     if (procp == NULL) {
@@ -51,5 +90,54 @@ md_proc_init(struct proc *procp, int flags)
         printf("md_proc_init: could not create new vas\n");
         return error;
     }
+
+    ds = USER_DS | 3;
+    cs = USER_CS | 3;
+
+    /*
+     * Set up the mapping specifier, we'll use zero
+     * to allocate new pages.
+     */
+    spec.pa = 0;
+    spec.va = STACK_TOP;
+
+    /* Put the trapframe in a known state */
+    tfp = &pcbp->tf;
+    memset(tfp, 0, sizeof(*tfp));
+    tfp->rflags = 0x202;
+    tfp->cs = cs;
+    tfp->ss = ds;
+
+    /* Map the stack */
+    vm_map(
+        &pcbp->vas, &spec,
+        STACK_LEN,
+        PROT_READ
+        | PROT_WRITE
+        | PROT_USER
+    );
+
+    tfp->rsp = STACK_TOP;
+    return 0;
+}
+
+/*
+ * Set process instruction pointer
+ */
+int
+md_set_ip(struct proc *procp, uintptr_t ip)
+{
+    struct md_pcb *pcbp;
+    struct trapframe *tfp;
+
+    if (procp == NULL) {
+        return -EINVAL;
+    }
+
+    pcbp = &procp->pcb;
+    tfp = &pcbp->tf;
+    tfp->rip = ip;
+    mmu_write_vas(&procp->pcb.vas);
+    __proc_kick(tfp);
     return 0;
 }
diff --git a/src/sys/include/sys/proc.h b/src/sys/include/sys/proc.h
index deed09b..d09e459 100644
--- a/src/sys/include/sys/proc.h
+++ b/src/sys/include/sys/proc.h
@@ -34,6 +34,12 @@
 #include <sys/queue.h>
 #include <machine/pcb.h>    /* standard */
 
+/*
+ * The stack starts here and grows down
+ */
+#define STACK_TOP   0xBFFFFFFF
+#define STACK_LEN   4096
+
 /*
  * A process describes a running program image
  * on the system.
@@ -69,4 +75,16 @@ int proc_init(struct proc *procp, int flags);
  */
 int md_proc_init(struct proc *procp, int flags);
 
+/*
+ * Set the instruction pointer field of a specific
+ * process
+ *
+ * @procp: Process to update
+ * @ip: Instruction pointer to write
+ *
+ * Returns zero on success, otherwise a less than
+ * zero value to indicate failure.
+ */
+int md_set_ip(struct proc *procp, uintptr_t ip);
+
 #endif  /* !_SYS_PROC_H_ */
-- 
cgit v1.2.3