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Date:   Fri, 9 Nov 2018 09:20:08 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH RFC 0/3] Static calls
Message-ID: <20181109152008.3z63vouyddtbeocu@treble>
References: <cover.1541711457.git.jpoimboe@redhat.com>
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On Fri, Nov 09, 2018 at 02:50:27PM +0100, Ard Biesheuvel wrote:
> On 9 November 2018 at 08:28, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> >> These patches are related to two similar patch sets from Ard and Steve:
> >>
> >> - https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@linaro.org
> >> - https://lkml.kernel.org/r/20181006015110.653946300@goodmis.org
> >>
> >> The code is also heavily inspired by the jump label code, as some of the
> >> concepts are very similar.
> >>
> >> There are three separate implementations, depending on what the arch
> >> supports:
> >>
> >>   1) CONFIG_HAVE_STATIC_CALL_OPTIMIZED: patched call sites - requires
> >>      objtool and a small amount of arch code
> >>
> >>   2) CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED: patched trampolines - requires
> >>      a small amount of arch code
> >>
> >>   3) If no arch support, fall back to regular function pointers
> >>
> >>
> >> TODO:
> >>
> >> - I'm not sure about the objtool approach.  Objtool is (currently)
> >>   x86-64 only, which means we have to use the "unoptimized" version
> >>   everywhere else.  I may experiment with a GCC plugin instead.
> >
> > I'd prefer the objtool approach. It's a pretty reliable first-principles
> > approach while GCC plugin would have to be replicated for Clang and any
> > other compilers, etc.
> >
> 
> I implemented the GCC plugin approach here for arm64
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=static-calls
>
> That implements both the unoptimized and the optimized versions.

Nice!  That was fast :-)

> I do take your point about GCC and other compilers, but on arm64 we
> don't have a lot of choice.
> 
> As far as I can tell, the GCC plugin is generic (i.e., it does not
> rely on any ARM specific passes, but obviously, this requires a *lot*
> of testing and validation to be taken seriously.

Yeah.  I haven't had a chance to try your plugin on x86 yet, but in
theory it should be arch-independent.

-- 
Josh