From 7184f527a0a880349a9697666c6d26db2d984706 Mon Sep 17 00:00:00 2001
From: Drew Galbraith <dgalbraith33@gmail.com>
Date: Mon, 29 May 2023 12:44:26 -0700
Subject: [PATCH] Jump to user mode.

This instantly creates a page fault as we always map pages with ring 0
permissions.
---
 zion/CMakeLists.txt              |  1 +
 zion/common/gdt.cpp              |  2 ++
 zion/common/gdt.h                |  4 ++++
 zion/loader/elf_loader.cpp       |  3 ++-
 zion/loader/elf_loader.h         |  3 ++-
 zion/scheduler/jump_user_space.s | 16 ++++++++++++++++
 zion/scheduler/thread.cpp        | 15 ++++++++++-----
 7 files changed, 37 insertions(+), 7 deletions(-)
 create mode 100644 zion/scheduler/jump_user_space.s

diff --git a/zion/CMakeLists.txt b/zion/CMakeLists.txt
index 4cf3ba1..2b35ec3 100644
--- a/zion/CMakeLists.txt
+++ b/zion/CMakeLists.txt
@@ -11,6 +11,7 @@ add_executable(zion
   memory/paging_util.cpp
   memory/physical_memory.cpp
   scheduler/context_switch.s
+  scheduler/jump_user_space.s
   scheduler/process.cpp
   scheduler/scheduler.cpp
   scheduler/thread.cpp
diff --git a/zion/common/gdt.cpp b/zion/common/gdt.cpp
index 2e0f7cf..b2f31a0 100644
--- a/zion/common/gdt.cpp
+++ b/zion/common/gdt.cpp
@@ -90,3 +90,5 @@ void InitGdt() {
       "ltr %%ax;" ::
           : "rax");
 }
+
+void SetRsp0(uint64_t rsp0) { gTaskStateSegment.rsp0 = rsp0; }
diff --git a/zion/common/gdt.h b/zion/common/gdt.h
index df0b364..5cee0fd 100644
--- a/zion/common/gdt.h
+++ b/zion/common/gdt.h
@@ -1,3 +1,7 @@
 #pragma once
 
+#include <stdint.h>
+
 void InitGdt();
+
+void SetRsp0(uint64_t rsp0);
diff --git a/zion/loader/elf_loader.cpp b/zion/loader/elf_loader.cpp
index 44474fe..2a7ed6a 100644
--- a/zion/loader/elf_loader.cpp
+++ b/zion/loader/elf_loader.cpp
@@ -56,7 +56,7 @@ void badmemcpy(uint64_t base, uint64_t offset, uint64_t dest) {
 
 }  // namespace
 
-void LoadElfProgram(uint64_t base, uint64_t offset) {
+uint64_t LoadElfProgram(uint64_t base, uint64_t offset) {
   Elf64Header* header = reinterpret_cast<Elf64Header*>(base);
   dbgln("phoff: %u phnum: %u", header->phoff, header->phnum);
   Elf64ProgramHeader* programs =
@@ -71,4 +71,5 @@ void LoadElfProgram(uint64_t base, uint64_t offset) {
     EnsureResident(program.vaddr, program.memsz);
     badmemcpy(base + program.offset, program.filesz, program.vaddr);
   }
+  return header->entry;
 }
diff --git a/zion/loader/elf_loader.h b/zion/loader/elf_loader.h
index 4feac5e..1c6c7e3 100644
--- a/zion/loader/elf_loader.h
+++ b/zion/loader/elf_loader.h
@@ -2,4 +2,5 @@
 
 #include <stdint.h>
 
-void LoadElfProgram(uint64_t base, uint64_t length);
+// Loads the elf program and returns its entry point.
+uint64_t LoadElfProgram(uint64_t base, uint64_t length);
diff --git a/zion/scheduler/jump_user_space.s b/zion/scheduler/jump_user_space.s
new file mode 100644
index 0000000..da077f9
--- /dev/null
+++ b/zion/scheduler/jump_user_space.s
@@ -0,0 +1,16 @@
+.global jump_user_space
+// rdi - entry point, rsi - user stack
+jump_user_space:
+  cli
+  mov $0x23, %ax
+  mov %ax, %ds
+  mov %ax, %es
+  mov %ax, %fs
+  mov %ax, %gs
+
+  pushq $0x23 # ss
+  pushq %rsi
+  pushf  # Can we just push 0 for flags?
+  pushq $0x1B # cs
+  pushq %rdi
+  iretq
diff --git a/zion/scheduler/thread.cpp b/zion/scheduler/thread.cpp
index 75753a5..fb4fef7 100644
--- a/zion/scheduler/thread.cpp
+++ b/zion/scheduler/thread.cpp
@@ -1,12 +1,16 @@
 #include "scheduler/thread.h"
 
+#include "common/gdt.h"
 #include "debug/debug.h"
 #include "loader/elf_loader.h"
+#include "memory/paging_util.h"
 #include "scheduler/process.h"
 #include "scheduler/scheduler.h"
 
 namespace {
 
+extern "C" void jump_user_space(uint64_t rip, uint64_t rsp);
+
 extern "C" void thread_init() {
   asm("sti");
   sched::CurrentThread().Init();
@@ -36,9 +40,10 @@ Thread::Thread(Process* proc, uint64_t tid, uint64_t elf_ptr)
 uint64_t Thread::pid() { return process_->id(); }
 
 void Thread::Init() {
-  LoadElfProgram(elf_ptr_, 0);
-  while (true) {
-    dbgln("[%u.%u]", pid(), id_);
-    sched::Yield();
-  }
+  dbgln("[%u.%u]", pid(), id_);
+  uint64_t rip = LoadElfProgram(elf_ptr_, 0);
+  uint64_t rsp = 0x80000000;
+  EnsureResident(rsp - 1, 1);
+  SetRsp0(rsp0_start_);
+  jump_user_space(rip, rsp);
 }