linux-kernel - Re: [PATCH 5/8] ftrace: Make DIRECT_CALLS work WITH_ARGS and !WITH

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Date:   Fri, 3 Feb 2023 10:03:38 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Florent Revest <revest@...omium.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kpsingh@...nel.org, jolsa@...nel.org,
        xukuohai@...weicloud.com
Subject: Re: [PATCH 5/8] ftrace: Make DIRECT_CALLS work WITH_ARGS and
 !WITH_REGS

On Thu, Feb 02, 2023 at 07:19:58PM +0100, Florent Revest wrote:
> On Thu, Feb 2, 2023 at 5:57 PM Mark Rutland <mark.rutland@....com> wrote:
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index 84f717f8959e..3d2156e335d7 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -241,6 +241,12 @@ enum {
> >         FTRACE_OPS_FL_DIRECT                    = BIT(17),
> >  };
> >
> > +#ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> > +#define FTRACE_OPS_FL_SAVE_ARGS                        FTRACE_OPS_FL_SAVE_REGS
> > +#else
> > +#define FTRACE_OPS_FL_SAVE_ARGS                        0
> 
> Mh, could we (theoretically) be in a situation where an arch supports
> WITH_ARGS but it also has two ftrace_caller trampolines: one that
> saves the args and the other that saves nothing ? (For example if x86
> migrates their WITH_REGS to WITH_ARGS only)

I don't think so -- the point of WITH_ARGS is that we always have to
save/restore the args, and when WITH_ARGS is selected they're unconditionally
available (though not necessarily a full pt_regs), which is what other code
assumes when WITH_ARGS is selected.

> Wouldn't it make sense then to define FTRACE_OPS_FL_SAVE_ARGS as an
> extra bit to tell ftrace that we need the args, similarly to
> FTRACE_OPS_FL_SAVE_REGS ?
> 
> If that can't happen or if we want to leave this up for later, the
> patch lgtm and I can squash it into my patch 5 in v2. ;)

I think that can't happen, and for now the above should be fine.

Thanks,
Mark.