linux-kernel - Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

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Message-ID: <7FA580EA-9DEA-4813-8832-F1BE034A857C@vmware.com>
Date:   Thu, 3 Jan 2019 18:30:08 +0000
From:   Nadav Amit <namit@...are.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

> On Jan 3, 2019, at 10:10 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> On Mon, Dec 31, 2018 at 07:53:06PM +0000, Nadav Amit wrote:
>>> On Dec 31, 2018, at 11:51 AM, Andy Lutomirski <luto@...nel.org> wrote:
>>> 
>>> On Sun, Dec 30, 2018 at 11:20 PM Nadav Amit <namit@...are.com> wrote:
>>>> This is a revised version of optpolines (formerly named retpolines) for
>>>> dynamic indirect branch promotion in order to reduce retpoline overheads
>>>> [1].
>>> 
>>> Some of your changelogs still call them "relpolines".
>>> 
>>> I have a crazy suggestion: maybe don't give them a cute name at all?
>>> Where it's actually necessary to name them (like in a config option),
>>> use a description like CONFIG_DYNAMIC_DEVIRTUALIZATION or
>>> CONFIG_PATCH_INDIRECT_CALLS or something.
> 
> Cute or not, naming is important.
> 
> If you want a description instead of a name, it will be a challenge to
> describe it in 2-3 words.
> 
> I have no idea what "dynamic devirtualization" means.
> 
> "Patch indirect calls" doesn't fully describe it either (and could be
> easily confused with static calls and some other approaches).
> 
>> I’m totally fine with that (don’t turn me into a "marketing” guy). It was
>> just a way to refer to the mechanism. I need more feedback about the more
>> fundamental issues to go on.
> 
> Naming isn't marketing.  It's a real issue: it affects both usability
> and code readability.

Well, allow me to be on the fence not this one.

I look for the path of least resistance. I think it would be easiest if I
first manage to make Josh’s static calls to be less intrusive. For that, I
try to add in the GCC plugin an attribute to annotate the function pointers
whose calls should be promoted. However, I don’t manage to get the
declaration from the call instruction's rtx. If anyone has a pointer on how
it can be done, that’s would be very helpful.