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Date:   Thu, 29 Nov 2018 14:12:09 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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On Thu, 29 Nov 2018 11:08:26 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, Nov 29, 2018 at 10:58 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > In contrast, if the call was wrapped in an inline asm, we'd *know* the
> > compiler couldn't turn a "call wrapper(%rip)" into anything else.  
> 
> Actually, I think I have a better model - if the caller is done with inline asm.
> 
> What you can do then is basically add a single-byte prefix to the
> "call" instruction that does nothing (say, cs override), and then
> replace *that* with a 'int3' instruction.
> 
> Boom. Done.
> 
> Now, the "int3" handler can just update the instruction in-place, but
> leave the "int3" in place, and then return to the next instruction
> byte (which is just the normal branch instruction without the prefix
> byte).
> 
> The cross-CPU case continues to work, because the 'int3' remains in
> place until after the IPI.
> 
> But that would require that we'd mark those call instruction with
> 

In my original proof of concept, I tried to to implement the callers
with asm, but then the way to handle parameters became a nightmare.

The goal of this (for me) was to replace the tracepoint indirect calls
with static calls, and tracepoints can have any number of parameters to
pass. I ended up needing the compiler to help me with the passing of
parameters.

-- Steve