linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Date:   Thu, 29 Nov 2018 14:31:33 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, mhiramat@...nel.org,
        jbaron@...mai.com, Jiri Kosina <jkosina@...e.cz>,
        David.Laight@...lab.com, bp@...en8.de, julia@...com,
        jeyu@...nel.org, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64

On Thu, 29 Nov 2018 11:24:43 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Nov 29, 2018 at 11:16 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > But then we need to implement all numbers of parameters.  
> 
> Oh, I agree, it's nasty.
> 
> But it's actually a nastiness that we've solved before. In particular,
> with the system call mappings, which have pretty much the exact same
> issue of "map unknown number of arguments to registers".
> 
> Yes, it's different - there you map the unknown number of arguments to
> a structure access instead. And yes, the macros are unbelievably ugly.
> See
> 
>     arch/x86/include/asm/syscall_wrapper.h

Those are not doing inline assembly.

> 
> and the __MAP() macro from
> 
>     include/linux/syscalls.h
> 
> so it's not pretty. But it would solve all the problems.
> 

Again, not inline assembly, and those only handle up to 6 parameters.

My POC started down this route, until I notice that there's tracepoints
that have 13 parameters! And I need to handle all tracepoints.

Yes, we can argue that we need to change those (if that doesn't break
the API of something using it).

-- Steve