/*
 * kernel_entry.S — kernel stack setup and entry trampoline (riscv64)
 *
 * UEFI hands control to kernel_main() running on EDK2's own boot-time stack.
 * Unlike amd64 (see arch/amd64/kernel_entry.S, which has carried this same
 * trampoline since early on), riscv64 previously called straight into
 * kernel_main_impl with no stack switch at all. The VM bootstrap's call
 * depth (27 chained word-registration modules, physics/SSM init, capsule
 * birth, Tripod fleet spawn) overflowed EDK2's riscv64 boot stack partway
 * through vm_init_with_host()'s return, corrupting the return address and
 * producing a wild jump shortly after — observed as a load/instruction
 * page fault (scause 0xd or 0xc) whose exact address drifted between
 * builds while the crash always landed in this same call region. amd64
 * never had this problem because kernel_entry.S already switches to a
 * dedicated 2 MiB BSS stack before anything deep runs; aarch64 apparently
 * gets away with EDK2's default boot stack being large enough, but that's
 * incidental, not a guarantee.
 *
 * kernel_main is the symbol the UEFI loader calls (extern in uefi_loader.c).
 * kernel_main_impl is defined in kernel_main.c and contains all the C code.
 *
 * Calling convention: RISC-V LP64D — boot_info pointer arrives in a0 and is
 * passed unchanged to kernel_main_impl via the tail-call.
 */

#include "starkernel/boot_info_offsets.h"

    /* 2 MiB kernel stack in BSS — fallback when no dynamic stack requested */
    .section .bss
    .align 4
    .global g_kernel_stack
g_kernel_stack:
    .space 0x200000
g_kernel_stack_top:
    .global g_kernel_stack_top

    .section .text
    .extern kernel_main_impl

    /*
     * kernel_main — stack-switch trampoline, then tail-call kernel_main_impl.
     *
     * a0 = BootInfo*  (RISC-V LP64D ABI — first argument register)
     *
     * Stack selection (checked in order):
     *   1. boot_info->kernel_stack_base != 0  →  use loader-allocated stack
     *      (top = base + size, aligned down to 16)
     *   2. Otherwise  →  fall back to g_kernel_stack (2 MiB BSS)
     *
     * Never returns; kernel_main_impl runs the REPL forever or panics.
     */
    .global kernel_main
kernel_main:
    ld   t0, BOOT_INFO_KERNEL_STACK_BASE_OFFSET(a0)
    bnez t0, .Ldynamic_stack

    /* BSS fallback */
    la   t0, g_kernel_stack_top
    j    .Lstack_ready

.Ldynamic_stack:
    ld   t1, BOOT_INFO_KERNEL_STACK_SIZE_OFFSET(a0)
    add  t0, t0, t1          /* top = base + size (stack grows down) */

.Lstack_ready:
    andi t0, t0, -16         /* 16-byte alignment (RISC-V psABI requirement) */
    mv   sp, t0
    mv   s0, zero            /* terminate frame-pointer chain */
    tail kernel_main_impl    /* a0 (boot_info) already in place */