boot/aarch64/entry.cpp


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// ============================================================================
// aarch64/entry.cpp - UEFI entry point for AArch64
//
// Kernel entry point: extern "C" [[noreturn]] void kernel_main(BootInfo*);
// Chain: UEFI -> efi_main -> jumps to _start (entry.S) -> calls kernel_main (from kernel_main.cpp)
// Why we need to run asm code:
// Because we haven't stack pointer, that needed for c/c++ 
// In efi_main / efi_loader_main we have it, because UEFI setup it for us
// ============================================================================

#include <boot/efi.h>
#include <boot/boot_info.h>

extern bool efi_loader_main(EFI_HANDLE image_handle, EFI_SYSTEM_TABLE* system_table, BootInfo* boot_info);

static BootInfo g_boot_info{};

typedef void (*KernelEntry)(BootInfo*); // _start

extern "C" EFI_STATUS EFIAPI efi_main(EFI_HANDLE image_handle, EFI_SYSTEM_TABLE* system_table) {
    if (!efi_loader_main(image_handle, system_table, &g_boot_info))
        return EFI_LOAD_ERROR;

    /*
        entry_offset - how far the entry point is from the start of the kernel image.
            For example if kernel_entry_point is 0x100000 and kernel_virt_base is 0x100000, the offset is 0
        phys_entry - the actual physical address to jump to: kernel_phys_base + entry_offset
    */
    uint64_t entry_offset = g_boot_info.kernel_entry_point - g_boot_info.kernel_virt_base;
    uint64_t phys_entry   = g_boot_info.kernel_phys_base + entry_offset;

    auto kernel_entry = reinterpret_cast<KernelEntry>(phys_entry);
    kernel_entry(&g_boot_info);  // X0 = &g_boot_info (AAPCS64)

    for (;;) asm volatile("wfi");
}