/* opencoreC - Thread Context Switch & Entry Trampoline */
.global switch_to
.global thread_entry_trampoline

.section .text
/*
 * switch_to(uint64_t *prev_rsp_ptr, uint64_t next_rsp)
 * RDI = &current_thread.context.rsp  (save current RSP here)
 * RSI = next thread's saved RSP
 */
switch_to:
    /* Save callee-saved registers of outgoing thread */
    push rbp
    push rbx
    push r12
    push r13
    push r14
    push r15

    /* Save RSP of outgoing thread */
    mov [rdi], rsp

    /* Load RSP of incoming thread */
    mov rsp, rsi

    /* Restore callee-saved registers of incoming thread */
    pop r15
    pop r14
    pop r13
    pop r12
    pop rbx
    pop rbp

    ret

/*
 * thread_entry_trampoline:
 * Jumped to via 'ret' in switch_to when a newly created thread runs for the first time.
 * When create_user_thread created the stack:
 *   R12 was populated with user_rsp
 *   R13 was populated with entry_rip
 */
thread_entry_trampoline:
    /* Set user data segments (0x18 | 3 = 0x1B) */
    mov ax, 0x1B
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    /* Build hardware IRETQ frame on kernel stack */
    push 0x1B          /* User SS */
    push r12           /* User RSP */
    push 0x3202        /* RFLAGS (IF=1, IOPL=3 for hardware drivers) */
    push 0x23          /* User CS (0x20 | 3 = 0x23) */
    push r13           /* User RIP */

    /* Clear all general-purpose registers */
    xor rax, rax
    xor rbx, rbx
    xor rcx, rcx
    xor rdx, rdx
    xor rsi, rsi
    xor rdi, rdi
    xor rbp, rbp
    xor r8,  r8
    xor r9,  r9
    xor r10, r10
    xor r11, r11
    xor r12, r12
    xor r13, r13
    xor r14, r14
    xor r15, r15

    /* Transition to Ring 3 */
    iretq