linux-kernel - Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement DYNAMIC_FTRACE_WITH

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Message-ID: <CADxym3b8+f2Gg-2RiK++9+D8yQ7yb44VQPnuHh2uS-CDRJjzEQ@mail.gmail.com>
Date: Sat, 15 Nov 2025 10:12:24 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, mhiramat@...nel.org, 
	mark.rutland@....com, mathieu.desnoyers@...icios.com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement DYNAMIC_FTRACE_WITH_JMP

On Sat, Nov 15, 2025 at 12:39 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 14 Nov 2025 17:24:45 +0800
> Menglong Dong <menglong8.dong@...il.com> 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
>
>