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Date:   Wed, 10 Oct 2018 15:16:09 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andrew Lutomirski <luto@kernel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature
 "jump_function"
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On Wed, Oct 10, 2018 at 02:56:08PM -0400, Steven Rostedt wrote:
> On Wed, 10 Oct 2018 13:33:30 -0500
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > Re-reading your suggestion, I may have misunderstood what you're
> > suggesting here, but I'm thinking about doing something like what you
> > proposed earlier:
> > 
> >     GLOBAL(tramp)
> >       jmp *current_func(%rip)
> >     ENDPROC(tramp)
> > 
> > That is, doing an indirect jump instead of the above direct jump, so
> > that any previous references to the trampoline would still work (and it
> > would also work during early boot).
> > 
> > Though it should probably be a retpoline instead of an indirect jump.
> 
> But do we care, as it only takes place during text_poke_bp() right?
> 
> I don't think we need to worry about training trampoline branch
> prediction that can only be hit when something enables the jump.

Yeah, I guess it depends on if we'd expect anybody (or gcc) to get a
function pointer to the trampoline itself.  I can just create a warning
for that in objtool.

-- 
Josh