linux-kernel - Re: [PATCH v4 14/18] static_call: Add static_cond

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Message-ID: <20200506161849.GV3762@hirez.programming.kicks-ass.net>
Date:   Wed, 6 May 2020 18:18:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     ndesaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        bristot <bristot@...hat.com>, jbaron <jbaron@...mai.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()

On Wed, May 06, 2020 at 10:01:12AM -0400, Mathieu Desnoyers wrote:
> ----- On May 6, 2020, at 9:55 AM, Peter Zijlstra peterz@...radead.org wrote:
> 
> > On Tue, May 05, 2020 at 04:27:44PM -0400, Mathieu Desnoyers wrote:
> >> Actually, if the goal is to do code patching of the call, I wonder
> >> what makes it OK to "guess" all the call patterns generated by the compiler ?
> > 
> > We're not guessing, have have objtool read the compiler output and
> > record the location for us. The compiler can generate whatever it likes.
> 
> So is the plan to adapt objtool if future compilers change the generated
> instruction patterns ?

If needed, sure. I don't really see what a compiler can do differently
though.

objtool looks for:

	JMP/CALL	__SCT__##foo

and writes a .static_call_sites table entry for it. Anything else we
can't rewrite anyway and will have to keep relying on the trampoline
working (which we also update, so that's fine).