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Date:   Wed, 6 May 2020 18:18:49 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     ndesaulniers <ndesaulniers@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        bristot <bristot@redhat.com>, jbaron <jbaron@akamai.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Nadav Amit <namit@vmware.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()
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On Wed, May 06, 2020 at 10:01:12AM -0400, Mathieu Desnoyers wrote:
> ----- On May 6, 2020, at 9:55 AM, Peter Zijlstra peterz@infradead.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).