1 files changed, 176 insertions, 0 deletions
diff --git a/boot/user.zig b/boot/user.zig
new file mode 100644
index 0000000..61345e5
--- /dev/null
+++ b/boot/user.zig
@@ -0,0 +1,176 @@
+// ring3 entry + the syscall boundary
+// drop user mode with iretq, and come back to the kernel
+//
+// see: - https://wiki.osdev.org/System_Calls 
+//      - https://wiki.osdev.org/Getting_to_Ring_3
+
+const serial = @import("serial.zig");
+const idt = @import("idt.zig");
+const vmm = @import("vmm.zig");
+const pmm = @import("pmm.zig");
+
+// where enterUserAsm parked the kernel stack
+export var kernel_return_rsp: u64 = 0;
+// args to enterUserAsm, passed via globals so it can stay a clean naked fn :flushed:
+export var g_user_entry: u64 = 0;
+export var g_user_stack: u64 = 0;
+
+// what the syscall stub leaves on the stack (GP regs it pushed, then the frame the CPU pushed on the int 0x80)
+// rax holds the syscall number
+pub const SyscallFrame = extern struct {
+    r15: u64,
+    r14: u64,
+    r13: u64,
+    r12: u64,
+    r11: u64,
+    r10: u64,
+    r9: u64,
+    r8: u64,
+    rbp: u64,
+    rdi: u64,
+    rsi: u64,
+    rdx: u64,
+    rcx: u64,
+    rbx: u64,
+    rax: u64,
+    rip: u64,
+    cs: u64,
+    rflags: u64,
+    rsp: u64,
+    ss: u64,
+};
+
+// install the int 0x80 gate at DPL 3 so ring3 code is allowed to invoke it.
+pub fn installSyscall() void {
+    idt.setGate(0x80, @intFromPtr(&syscallStub), 3);
+}
+
+// drop to ring 3 at g_user_entry with stack g_user_stack
+// returns (via the exit syscall s longjmp) once the program calls exit
+pub fn enterUser(entry: u64, ustack: u64) void {
+    g_user_entry = entry;
+    g_user_stack = ustack;
+    // this is the truest most correct statement ever
+    // get this man a true
+    asm volatile ("call enterUserAsm" ::: .{ 
+        .memory = true, 
+        .rax = true, 
+        .rcx = true, 
+        .rdx = true, 
+        .rsi = true, 
+        .rdi = true, 
+        .r8 = true, 
+        .r9 = true, 
+        .r10 = true, 
+        .r11 = true });
+}
+
+export fn enterUserAsm() callconv(.naked) void {
+    asm volatile (
+        \\ push %rbx
+        \\ push %rbp
+        \\ push %r12
+        \\ push %r13
+        \\ push %r14
+        \\ push %r15
+        \\ mov %rsp, kernel_return_rsp(%rip)
+        \\ pushq $0x23              // SS = user data (0x20) | RPL 3
+        \\ mov g_user_stack(%rip), %rax
+        \\ push %rax                // user RSP
+        \\ pushq $0x2               // RFLAGS: reserved bit only (IF=0 for the demo)
+        \\ pushq $0x1b              // CS = user code (0x18) | RPL 3
+        \\ mov g_user_entry(%rip), %rax
+        \\ push %rax                // RIP = entry
+        \\ iretq
+    );
+}
+
+export fn syscallStub() callconv(.naked) void {
+    asm volatile (
+        \\ push %rax
+        \\ push %rbx
+        \\ push %rcx
+        \\ push %rdx
+        \\ push %rsi
+        \\ push %rdi
+        \\ push %rbp
+        \\ push %r8
+        \\ push %r9
+        \\ push %r10
+        \\ push %r11
+        \\ push %r12
+        \\ push %r13
+        \\ push %r14
+        \\ push %r15
+        \\ mov %rsp, %rdi
+        \\ call syscallHandler       // rax = 0 continue, 1 exit
+        \\ test %rax, %rax
+        \\ jnz 1f
+        \\ pop %r15
+        \\ pop %r14
+        \\ pop %r13
+        \\ pop %r12
+        \\ pop %r11
+        \\ pop %r10
+        \\ pop %r9
+        \\ pop %r8
+        \\ pop %rbp
+        \\ pop %rdi
+        \\ pop %rsi
+        \\ pop %rdx
+        \\ pop %rcx
+        \\ pop %rbx
+        \\ pop %rax
+        \\ iretq
+        \\ 1:
+        \\ mov kernel_return_rsp(%rip), %rsp
+        \\ pop %r15
+        \\ pop %r14
+        \\ pop %r13
+        \\ pop %r12
+        \\ pop %rbp
+        \\ pop %rbx
+        \\ ret
+    );
+}
+
+// returns 0 to resume the program, 1 to return to the kernel (exit).
+export fn syscallHandler(frame: *SyscallFrame) callconv(.c) u64 {
+    switch (frame.rax) {
+        0 => { // exit
+            serial.write("[syscall] exit -> back to kernel\r\n");
+            return 1;
+        },
+        1 => { // "hello" -- a fixed message, no user pointer to validate yet
+            serial.write("[syscall] hello from ring 3!\r\n");
+            return 0;
+        },
+        else => {
+            serial.write("[syscall] unknown\r\n");
+            return 0;
+        },
+    }
+}
+
+//   mov $1,%eax   ; int $0x80 (hello)
+//   xor %eax,%eax ; int $0x80 (exit)
+pub fn runTestProgram() void {
+    const as = vmm.createAddressSpace() orelse return;
+    const code_frame = pmm.alloc() orelse return;
+    const stack_frame = pmm.alloc() orelse return;
+    const code_virt: u64 = 0x400000; // 4 MiB
+    const stack_virt: u64 = 0x800000; // 8 MiB
+    _ = vmm.map(as, code_virt, code_frame, vmm.WRITE | vmm.USER);
+    _ = vmm.map(as, stack_virt, stack_frame, vmm.WRITE | vmm.USER);
+
+    const code = [_]u8{ 0xB8, 0x01, 0x00, 0x00, 0x00, 0xCD, 0x80, 0x31, 0xC0, 0xCD, 0x80 };
+    const dst: [*]u8 = @ptrFromInt(pmm.physToVirt(code_frame)); // write via HHDM
+    @memcpy(dst[0..code.len], &code);
+
+    const prev = vmm.activePml4();
+    vmm.loadPml4(as);
+    serial.write("[user] entering ring 3...\r\n");
+    enterUser(code_virt, stack_virt + pmm.PAGE_SIZE);
+    vmm.loadPml4(prev);
+    serial.write("[user] back in the kernel, continuing\r\n");
+}