linux-kernel - Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

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Message-ID: <20181109190320.tn3fpaegg5dkgvdb@treble>
Date:   Fri, 9 Nov 2018 13:03:20 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

On Fri, Nov 09, 2018 at 06:53:21PM +0100, Ard Biesheuvel wrote:
> > It's a bit fiddly since inline and out-of-line both use
> > arch_static_call_transform(), but what I need to do is basically:
> >
> > - for out-of-line, the trampoline needs to be patched into a
> > movn/movk/movk/br sequence if the target is too far
> > - for inline, the trampoline itself needs to be patched from
> > adrp/ldr/br (which does a load and a indirect branch) to
> > movn/movk/movk/br (which uses immediates), and the call sites need to
> > be patched into calls to the veneer if the target is out of range.
> >
> > So arch_static_call_transform() needs to know where the trampoline is
> > for this use case, so could we perhaps add a 'void *orig' in the key
> > struct that keeps track of the original value of 'addr' ?
> 
> ... and pass it to arch_static_call_transform

Ok.  So in both cases, you need both the call site address and the tramp
address, right?  We could add a 'tramp' pointer to the key struct -- I
assume that's what you mean?

And also change

  void arch_static_call_transform(unsigned long insn, void *dest)

to

  void arch_static_call_transform(void *call_site, void *tramp, void *dest)

-- 
Josh