[RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines

[Mark Rutland]

On Wed, May 25, 2022 at 09:43:07AM -0400, Steven Rostedt wrote:
> On Wed, 25 May 2022 13:17:30 +0100
> Mark Rutland <mark.rutland at arm.com> wrote:
> 
> > For arm64 I'd like to make this static, and have ftrace *always* capture a
> > minimal set of ftrace_regs, which would be:
> > 
> >   X0 to X8 inclusive
> >   SP
> >   PC
> >   LR
> >   FP
> > 
> > Since X0 to X8 + SP is all that we need for arguments and return values (per
> > the calling convention we use), and PC+LR+FP gives us everything we need for
> > unwinding and live patching.
> > 
> > I *might* want to add x18 to that when SCS is enabled, but I'm not immediately
> > sure.
> 
> Does arm64 have HAVE_DYNAMIC_FTRACE_WITH_ARGS enabled?

Not yet. I'd like to implement it, but always only saving the values above and
never saving a full pt_regs (since as mentioned elsewhere we can't do that
correctly anyway).

> If so, then having the normal ftrace call back save the above so that all
> functions have it available would be useful.

I think that's what I'm saying: I'd want to have one trampoline which always
saved the above, so all functions would get that.

Thanks,
Mark.