linux-kernel - Re: [PATCH v2] riscv/ftrace: Add basic support

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Date:   Tue, 12 Dec 2017 19:34:20 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alan Kao <nonerkao@...il.com>
Cc:     Palmer Dabbelt <palmer@...ive.com>, Jim Wilson <jimw@...ive.com>,
        albert@...ive.com, mingo@...hat.com, patches@...ups.riscv.org,
        linux-kernel@...r.kernel.org, greentime@...estech.com,
        alankao@...estech.com, pombredanne@...b.com, kito@...estech.com
Subject: Re: [PATCH v2] riscv/ftrace: Add basic support

On Tue, 12 Dec 2017 15:08:00 +0800
Alan Kao <nonerkao@...il.com> wrote:

> > It's not a big deal, though -- we can fix these later.  The more interesting
> > thing here is that this code means our `-pg` stuff is now part of the GCC
> > ABI, which is something I'd never though of before.  I've added Jim, our GCC
> > guy.
> > 
> > Jim: do you mind checking to make sure the GCC profiling support is sane?
> > Specifically, I'm thinking:
> > 
> > * Are there any profiling features we don't support that would require an
> > ABI break?
> > * Is there a way to add future ISA extensions without breaking the ABI?
> > * Should we document this as part of the ELF psABI specification?
> > 
> > Even though this isn't user-visible as far an Linux is concerned, it'd be a
> > bit of a pain to have to break this ABI because we did something brain-dead.
> > Since there's a bit of time before 7.3.0, I think it'd be OK to consider
> > breaking the profiling ABI if there's a good reason.
> >   
> 
> As far as I can tell, the `-pg` flag only inserts the _mcount call after every 
> valid function prologue and seems breaking no existing ABI. But indeed
> it would be good if compiler guys can take a look at the gcc profiling
> features.

This is an interesting discussion, although I'm a bit confused. What
ABI are you worried about breaking?

-- Steve