Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement DYNAMIC_FTRACE_WITH_JMP

[Menglong Dong]

On Sat, Nov 15, 2025 at 12:39 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Fri, 14 Nov 2025 17:24:45 +0800
> Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> > --- a/arch/x86/kernel/ftrace_64.S
> > +++ b/arch/x86/kernel/ftrace_64.S
> > @@ -285,8 +285,18 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
> >       ANNOTATE_NOENDBR
> >       RET
> >
> > +1:
> > +     testb   $1, %al
> > +     jz      2f
> > +     andq $0xfffffffffffffffe, %rax
> > +     movq %rax, MCOUNT_REG_SIZE+8(%rsp)
> > +     restore_mcount_regs
> > +     /* Restore flags */
> > +     popfq
> > +     RET
> > +
> >       /* Swap the flags with orig_rax */
> > -1:   movq MCOUNT_REG_SIZE(%rsp), %rdi
> > +2:   movq MCOUNT_REG_SIZE(%rsp), %rdi
> >       movq %rdi, MCOUNT_REG_SIZE-8(%rsp)
> >       movq %rax, MCOUNT_REG_SIZE(%rsp)
> >
>
> So in this case we have:
>
>  original_caller:
>  call foo -> foo:
>              call fentry -> fentry:
>                             [do ftrace callbacks ]
>                             move tramp_addr to stack
>                             RET -> tramp_addr
>                                             tramp_addr:
>                                             [..]
>                                             call foo_body -> foo_body:
>                                                              [..]
>                                                              RET -> back to tramp_addr
>                                             [..]
>                                             RET -> back to original_caller

Nice flow chart, which I think we can put in the commit log.

>
> I guess that looks balanced.

Yes, it is balanced.

>
> -- Steve
>
>