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Date:   Tue, 7 May 2019 19:38:29 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        the arch/x86 maintainers <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        Miroslav Benes <mbenes@suse.cz>,
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Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
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On Tue, May 07, 2019 at 10:08:50AM -0700, Linus Torvalds wrote:
> On Tue, May 7, 2019 at 9:34 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Would you consider my approach later on, under the guise of unification?
> 
> WHY?
> 
> The *only* advantage of your patch is that trivial "look up kernel stack" macro.
> 
> Seriously. There's absolutely nothing else.

The ftrace_regs_caller, the kprobe tramplines, the unwinder, they all
have 'funny' bits because pt_regs isn't 'right'.

> So the whole "let's clean up x86-32 to look like x86-64, which got
> things right" is to me a completely bogus argument. x86-64 got the
> "yes, push ss/sp unconditionally" part right, but got a lot of other
> things horribly wrong. So this is all just one small detail that
> differs, across two architectures that are similar but have very
> different warts.

It's a detail that leaks into the C code. Yes SWAPGS is horrible crap,
but C code doesn't much care. The partial pt_regs thing otoh comes up a
fair number of times.

Anyway; I think we're at the point where we'll have to agree to
disagree (or maybe slightly past it).