linux-kernel - Re: [PATCH 10/15] static_call: Add basic static call infrastructure

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Date:   Fri, 7 Jun 2019 09:33:27 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <namit@...are.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.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>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 10/15] static_call: Add basic static call infrastructure



> On Jun 7, 2019, at 1:49 AM, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> 
>> On Fri, 7 Jun 2019 at 10:29, Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> On Thu, Jun 06, 2019 at 10:44:23PM +0000, Nadav Amit wrote:
>>>> + * Usage example:
>>>> + *
>>>> + *   # Start with the following functions (with identical prototypes):
>>>> + *   int func_a(int arg1, int arg2);
>>>> + *   int func_b(int arg1, int arg2);
>>>> + *
>>>> + *   # Define a 'my_key' reference, associated with func_a() by default
>>>> + *   DEFINE_STATIC_CALL(my_key, func_a);
>>>> + *
>>>> + *   # Call func_a()
>>>> + *   static_call(my_key, arg1, arg2);
>>>> + *
>>>> + *   # Update 'my_key' to point to func_b()
>>>> + *   static_call_update(my_key, func_b);
>>>> + *
>>>> + *   # Call func_b()
>>>> + *   static_call(my_key, arg1, arg2);
>>> 
>>> I think that this calling interface is not very intuitive.
>> 
>> Yeah, it is somewhat unfortunate..
>> 
> 
> Another thing I brought up at the time is that it would be useful to
> have the ability to 'reset' a static call to its default target. E.g.,
> for crypto modules that implement an accelerated version of a library
> interface, removing the module should revert those call sites back to
> the original target, without putting a disproportionate burden on the
> module itself to implement the logic to support this.

I was thinking this could be a layer on top.  We could have a way to register a static call with the module core so that, when a GPL module with an appropriate symbol is loaded, the static call gets replaced.

KVM could use this too.  Or we could just require KVM to be built in some day.