use core::arch::asm;
use x86_64::registers::rflags::RFlags;

pub struct EnterUserModeInput {
    pub rip: u64,
    pub rsp: u64,
    pub rflags: RFlags
}

/// # Safety
/// Does `sysretq`
/// Make sure that you are not letting the user space program do things you
/// don't want it to do. You must enable system call extensions first.
pub unsafe fn enter_user_mode(EnterUserModeInput { rip, rsp, rflags }: EnterUserModeInput) -> ! {
    let rflags = rflags.bits();
    unsafe {
        // Note that we do `sysretq` and not `sysret` because if we just do
        // `sysret`, that could be compiled into a `sysretl`, which is for
        // 32-bit compatibility mode and can mess things up.
        asm!("\
            mov rsp, {}
            sysretq",
            in(reg) rsp,
            in("rcx") rip,
            in("r11") rflags,
            // The user space program can only "return" with a `syscall`, whic
            // will jump to the syscall handler
            options(noreturn)
        );
    }
}