From 79aeff9cee62b8918be53b4607f5579079c7fb3e Mon Sep 17 00:00:00 2001
From: Putta Khunchalee <ultimaweapon@outlook.com>
Date: Sun, 22 Sep 2024 02:16:05 +0700
Subject: [PATCH] Switches from VMM GDT to kernel GDT (#992)

---
 src/obkrnl/Cargo.toml     |  1 +
 src/obkrnl/src/aarch64.rs |  3 ++
 src/obkrnl/src/main.rs    | 15 ++++--
 src/obkrnl/src/x86_64.rs  | 96 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 110 insertions(+), 5 deletions(-)
 create mode 100644 src/obkrnl/src/aarch64.rs
 create mode 100644 src/obkrnl/src/x86_64.rs

diff --git a/src/obkrnl/Cargo.toml b/src/obkrnl/Cargo.toml
index 3a79cae98..3b8e5ed3b 100644
--- a/src/obkrnl/Cargo.toml
+++ b/src/obkrnl/Cargo.toml
@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 anstyle = { version = "1.0.8", default-features = false }
+bitfield-struct = "0.8.0"
 hashbrown = "0.14.5"
 macros = { path = "../macros" }
 obconf = { path = "../obconf" }
diff --git a/src/obkrnl/src/aarch64.rs b/src/obkrnl/src/aarch64.rs
new file mode 100644
index 000000000..2ae06f2e8
--- /dev/null
+++ b/src/obkrnl/src/aarch64.rs
@@ -0,0 +1,3 @@
+pub unsafe fn setup_main_cpu() {
+    todo!()
+}
diff --git a/src/obkrnl/src/main.rs b/src/obkrnl/src/main.rs
index 9224317f7..c18c578a7 100644
--- a/src/obkrnl/src/main.rs
+++ b/src/obkrnl/src/main.rs
@@ -10,6 +10,9 @@ use core::mem::zeroed;
 use core::panic::PanicInfo;
 use obconf::{BootEnv, Config};
 
+#[cfg_attr(target_arch = "aarch64", path = "aarch64.rs")]
+#[cfg_attr(target_arch = "x86_64", path = "x86_64.rs")]
+mod arch;
 mod config;
 mod console;
 mod context;
@@ -37,14 +40,16 @@ extern crate alloc;
 /// See PS4 kernel entry point for a reference.
 #[allow(dead_code)]
 #[cfg_attr(target_os = "none", no_mangle)]
-extern "C" fn _start(env: &'static BootEnv, conf: &'static Config) -> ! {
+unsafe extern "C" fn _start(env: &'static BootEnv, conf: &'static Config) -> ! {
     // SAFETY: This function has a lot of restrictions. See Context documentation for more details.
-    unsafe { crate::config::setup(env, conf) };
+    crate::config::setup(env, conf);
 
     info!("Starting Obliteration Kernel.");
 
+    self::arch::setup_main_cpu();
+
     // Setup thread0 to represent this thread.
-    let thread0 = unsafe { Thread::new_bare() };
+    let thread0 = Thread::new_bare();
 
     // Initialize foundations.
     let pmgr = ProcMgr::new();
@@ -52,9 +57,9 @@ extern "C" fn _start(env: &'static BootEnv, conf: &'static Config) -> ! {
     // Activate CPU context. We use a different mechanism here. The PS4 put all of pcpu at a global
     // level but we put it on each CPU stack instead.
     let thread0 = Arc::new(thread0);
-    let mut cx = unsafe { Context::new(0, thread0, pmgr.clone()) };
+    let mut cx = Context::new(0, thread0, pmgr.clone());
 
-    unsafe { cx.activate() };
+    cx.activate();
 
     main(pmgr);
 }
diff --git a/src/obkrnl/src/x86_64.rs b/src/obkrnl/src/x86_64.rs
new file mode 100644
index 000000000..11e6c631b
--- /dev/null
+++ b/src/obkrnl/src/x86_64.rs
@@ -0,0 +1,96 @@
+use bitfield_struct::bitfield;
+use core::arch::global_asm;
+
+pub unsafe fn setup_main_cpu() {
+    // Switch GDT from bootloader GDT to our own.
+    static GDT: [SegmentDescriptor; 3] = [
+        // Null descriptor.
+        SegmentDescriptor::new(),
+        // Code segment.
+        SegmentDescriptor::new()
+            .with_ty(0b1000) // This required somehow although the docs said it is ignored.
+            .with_s(true) // Same here.
+            .with_p(true)
+            .with_l(true), // 64-bit mode.
+        // Data segment.
+        SegmentDescriptor::new()
+            .with_ty(0b0010) // This required somehow although the docs said it is ignored.
+            .with_s(true) // Same here.
+            .with_p(true),
+    ];
+
+    set_gdtr(
+        &Gdtr {
+            limit: (size_of_val(&GDT) - 1).try_into().unwrap(),
+            addr: GDT.as_ptr(),
+        },
+        SegmentSelector::new().with_si(1),
+        SegmentSelector::new().with_si(2),
+    );
+}
+
+extern "C" {
+    fn set_gdtr(v: &Gdtr, code: SegmentSelector, data: SegmentSelector);
+}
+
+// See lgdt on the PS4 for a reference.
+global_asm!(
+    "set_gdtr:",
+    "lgdt qword ptr [rdi]",
+    "mov ds, dx",
+    "mov es, dx",
+    "mov fs, dx",
+    "mov gs, dx",
+    "mov ss, dx",
+    "pop rax",  // Return address.
+    "push rsi", // Code segment selector.
+    "push rax",
+    "retfq" // Set CS then return.
+);
+
+/// Raw value of a Global Descriptor-Table Register.
+///
+/// See Global Descriptor-Table Register section on AMD64 Architecture Programmer's Manual Volume 2
+/// for details.
+#[repr(C, packed)]
+struct Gdtr {
+    limit: u16,
+    addr: *const SegmentDescriptor,
+}
+
+/// Raw value of a Segment Descriptor.
+///
+/// See Legacy Segment Descriptors section on AMD64 Architecture Programmer's Manual Volume 2 for
+/// more details.
+#[bitfield(u64)]
+struct SegmentDescriptor {
+    limit1: u16,
+    #[bits(24)]
+    base1: u32,
+    #[bits(4)]
+    ty: u8,
+    s: bool,
+    #[bits(2)]
+    dpl: u8,
+    p: bool,
+    #[bits(4)]
+    limit2: u8,
+    avl: bool,
+    l: bool,
+    db: bool,
+    g: bool,
+    base2: u8,
+}
+
+/// Raw value of a Segment Selector (e.g. `CS` and `DS` register).
+///
+/// See Segment Selectors section on AMD64 Architecture Programmer's Manual Volume 2 for more
+/// details.
+#[bitfield(u16)]
+struct SegmentSelector {
+    #[bits(2)]
+    rpl: u8,
+    ti: bool,
+    #[bits(13)]
+    si: u16,
+}