linux-kernel - Re: [PATCH 2/4] objtool: Implement jump_assert for _static_cpu

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Message-ID: <20180115180405.cdzdgw6tocbsmbrf@treble>
Date:   Mon, 15 Jan 2018 12:04:05 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 2/4] objtool: Implement jump_assert for _static_cpu_has()

On Mon, Jan 15, 2018 at 05:44:30PM +0100, Peter Zijlstra wrote:
> Unlike the jump_label bits, static_cpu_has is implemented with
> alternatives. Sadly it doesn't readily distinguish itself from any
> other alternatives.
> 
> Use a heuristic to guess at it :/
> 
> But like jump_labels, make static_cpu_has set br_static on the
> instructions after the static branch such that we can assert on it.

This seems a bit heavy handed and fragile, though maybe it is the best
way.  Still I wonder if there's a better way to do it.

Some quick ideas:

a) Somehow use __jump_table in the _static_cpu_has() macro?

b) Add another special annotation to tell objtool where
_static_cpu_has() locations are?

May need to ruminate on this one a bit...

-- 
Josh