linux-kernel - Re: [PATCH v2 00/13] Add static

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Message-ID: <20191007113307.GC2311@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Oct 2019 13:33:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com
Subject: Re: [PATCH v2 00/13] Add static_call()

On Mon, Oct 07, 2019 at 10:27:08AM +0200, Peter Zijlstra wrote:
> This series, which depends on the previous two, introduces static_call().
> 
> static_call(), is the idea of static_branch() applied to indirect function
> calls. Remove a data load (indirection) by modifying the text.
> 
> These patches are still based on the work Josh did earlier, but incorporated
> feedback from the last posting and have a lot of extra patches which resulted
> from me trying to actually use static_call().
> 
> This still relies on objtool to generate the .static_call_sites section, mostly
> because this is a natural place for x86_64 and works for both GCC and LLVM.
> Other architectures can pick other means if/when they implement the inline
> patching. The out-of-line (aka. trampoline) variant doesn't require this.
> 

FWIW, Steve, if we were to do something like:

CFLAGS += -mfentry -mfentry_name=____static_call_fentry_tramp

And have:

struct static_call_key ____static_call_fentry = {
	.func = NULL,
	.next = 1,
};
asm(".pushsection .static_call.text, \"ax\" \n"
    ".align 4 \n"
    ".globl ____static_call_fentry_tramp \n"
    "____static_call_fentry_tramp: \n"
    "  ret \n"
    ".type ____static_call_fentry_tramp, @function \n"
    ".size ____static_call_fentry_tramp, . - ____static_call_fentry_tramp \n"
    ".popsection \n");

Then the whole function entry thing would automagicaly turn into
something like static_cond_call(fentry)(...);

Not saying we should do that, but we could ;-)