Re: [RFC PATCH v2 2/6] tracing: Expose ftrace_regs regardless of CONFIG_FUNCTION_TRACER

[Florent Revest]

On Wed, Aug 9, 2023 at 4:16 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Wed, 9 Aug 2023 12:29:13 +0200
> Florent Revest <revest@xxxxxxxxxxxx> wrote:
> > > + * tracer, define the default(pt_regs compatible) ftrace_regs.
> > > + */
> > > +#if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || !defined(CONFIG_FUNCTION_TRACER)
> >
> > I wonder if we should make things simpler with:
> >
> > #if defined(HAVE_PT_REGS_COMPAT_FTRACE_REGS) || !defined(CONFIG_FUNCTION_TRACER)
> >
> > And remove the ftrace_regs definitions that are copy-pastes of this
> > block in arch specific headers. Then we can enforce in a single point
> > that HAVE_PT_REGS_COMPAT_FTRACE_REGS holds.
>
> Here, the "HAVE_PT_REGS_COMPAT_FTRACE_REGS" does not mean that the
> ftrace_regs is completely compatible with pt_regs, but on the memory
> it wraps struct pt_regs (thus we can just cast the type).

But in practice I think that all architectures that chose to wrap a
pt_regs in their ftrace_regs also do:

+#define ftrace_regs_get_instruction_pointer(fregs) \
+       instruction_pointer(ftrace_get_regs(fregs))
+#define ftrace_regs_get_argument(fregs, n) \
+       regs_get_kernel_argument(ftrace_get_regs(fregs), n)
+#define ftrace_regs_get_stack_pointer(fregs) \
+       kernel_stack_pointer(ftrace_get_regs(fregs))
+#define ftrace_regs_return_value(fregs) \
+       regs_return_value(ftrace_get_regs(fregs))
+#define ftrace_regs_set_return_value(fregs, ret) \
+       regs_set_return_value(ftrace_get_regs(fregs), ret)
+#define ftrace_override_function_with_return(fregs) \
+       override_function_with_return(ftrace_get_regs(fregs))
+#define ftrace_regs_query_register_offset(name) \
+       regs_query_register_offset(name)

And are just careful to populate the fields that let these macros
work. So maybe these could be factorized... But anyway, I'm not
particularly a super fan of the idea and I don't think it should
necessarily fit in that series. It's just something that crossed my
mind, if you're not a fan then we should probably not do it ;)