arch/x86/kernel/head_64.S

The startup_64 «function» contains the code that is first executed in the kernel when it is started.

This location is specified with the ENTRY() command in the linker script arch/$SRCARCH/kernel/vmlinux.lds.

This function is called either

• by a 64-bit bootloader, or
• from arch/x86/boot/compressed/head_64.S

But… see also

• the ENTRY symbol defined in arch/x86/kernel/vmlinux.lds.S
• the function startup_32, defined in arch/x86/kernel/head_32.S

arch/x86/kernel/head_32.S

arch/x86/boot/compressed/head_64.S