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Date:   Fri, 9 Nov 2018 08:28:11 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@ACULAB.COM>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH RFC 0/3] Static calls
Message-ID: <20181109072811.GB86700@gmail.com>
References: <cover.1541711457.git.jpoimboe@redhat.com>
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* Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> These patches are related to two similar patch sets from Ard and Steve:
> 
> - https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@linaro.org
> - https://lkml.kernel.org/r/20181006015110.653946300@goodmis.org
> 
> The code is also heavily inspired by the jump label code, as some of the
> concepts are very similar.
> 
> There are three separate implementations, depending on what the arch
> supports:
> 
>   1) CONFIG_HAVE_STATIC_CALL_OPTIMIZED: patched call sites - requires
>      objtool and a small amount of arch code
>   
>   2) CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED: patched trampolines - requires
>      a small amount of arch code
>   
>   3) If no arch support, fall back to regular function pointers
> 
> 
> TODO:
> 
> - I'm not sure about the objtool approach.  Objtool is (currently)
>   x86-64 only, which means we have to use the "unoptimized" version
>   everywhere else.  I may experiment with a GCC plugin instead.

I'd prefer the objtool approach. It's a pretty reliable first-principles 
approach while GCC plugin would have to be replicated for Clang and any 
other compilers, etc.

> - Does this feature have much value without retpolines?  If not, should
>   we make it depend on retpolines somehow?

Paravirt patching, as you mention in your later reply?

> - Find some actual users of the interfaces (tracepoints? crypto?)

I'd be very happy with a demonstrated paravirt optimization already - 
i.e. seeing the before/after effect on the vmlinux with an x86 distro 
config.

All major Linux distributions enable CONFIG_PARAVIRT=y and 
CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much 
as possible in the 99.999% cases where it's not used is a primary 
concern.

All other usecases are bonus, but it would certainly be interesting to 
investigate the impact of using these APIs for tracing: that too is a 
feature enabled everywhere but utilized only by a small fraction of Linux 
users - so literally every single cycle or instruction saved or hot-path 
shortened is a major win.

Thanks,

	Ingo