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

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Message-ID: <CAKv+Gu_REsDKZ4q4xSTTO8y7ZMMYd6kormb5NE5D-eV=GnwcZg@mail.gmail.com>
Date:   Fri, 9 Nov 2018 20:12:26 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 9 November 2018 at 20:03, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 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?
>

Yes.

> 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)
>

Indeed.