Re: [RFC PATCH 24/32] x86/ftrace: Enable HAVE_FUNCTION_GRAPH_FREGS

[Steven Rostedt]

On Mon, 6 Nov 2023 00:17:34 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> Changelog nor code made it clear this was partial anything. So this is
> still the partial thing?
> 
> Can we then pretty clear clarify all that, and make it clear which regs
> are in there? Because when I do 'vim -t ftrace_regs' it just gets me a
> seemingly pointless wrapper struct, no elucidating comments nothingses.

I agree it should be better documented (like everything else). The
ftrace_regs must have all the registers needed to produce a function's
arguments. For x86_64, that would be:

  rdi, rsi, rdx, r8, r9, rsp

Basically anything that is needed to call mcount/fentry.

But yes, it's still partial registers but for archs that support
FTRACE_WITH_REGS, it can also hold all pt_regs which can be retrieved
by the arch_ftrace_get_regs(), which is why there's a pt_regs struct in
the x86 version. But that's not the case for arm64, as
arch_ftrace_get_regs() will always return NULL.

> 
> > You even Acked the patch:
> >
> > commit 02a474ca266a47ea8f4d5a11f4ffa120f83730ad
> > Author: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> > Date:   Tue Oct 27 10:55:55 2020 -0400  
> 
> You expect me to remember things from 3 years ago?

Heh, of course not. I just thought it amusing that I created
ftrace_regs because of you and then 3 years later you ask to get rid of
it. But the real issue is that it's not documented clearly why it
exists, and that should be rectified.

Thanks,

-- Steve