I want to call printk from arch/x86/lib/copy_user_64.S. The only example in assembly that I can find for calling something like printk is from: arch/x86/kernel/head_64.S (code follows) here is what I tried: ENTRY(jj_print) CFI_STARTPROC ASM_STAC leaq jj_buf(%rip),%rdi mov $0, %rsi call printk xorl %eax,%eax ASM_CLAC ret CFI_ENDPROC ENDPROC(jj_print) jj_buf: .ascii "hello asm\n\0" Then I added this to where I wanted to run the code: ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,X86_FEATURE_ERMS, \ jj_print, jj_print, jj_print I wasn't sure how to make the call to jj_print so I just used the ALTERNATIVE_JUMP without trying to understand it, but from what I can tell, it seems like it should work (I know it is not ideal and lame) sample workig code which I based my jj_print func on: #ifdef CONFIG_EARLY_PRINTK GET_CR2_INTO(%r9) # can clobber any volatile register if pv movl 80(%rsp),%r8d # error code movl 72(%rsp),%esi # vector number movl 96(%rsp),%edx # %cs movq 88(%rsp),%rcx # %rip xorl %eax,%eax leaq early_idt_msg(%rip),%rdi call early_printk cmpl $2,early_recursion_flag(%rip) jz 1f call dump_stack #ifdef CONFIG_KALLSYMS leaq early_idt_ripmsg(%rip),%rdi movq 40(%rsp),%rsi # %rip again call __print_symbol #endif #endif /* EARLY_PRINTK */