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Date: Mon, 15 Jan 2018 12:04:05 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@infradead.org>,
        linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
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        Andi Kleen <ak@linux.intel.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 2/4] objtool: Implement jump_assert for _static_cpu_has()
Message-ID: <20180115180405.cdzdgw6tocbsmbrf@treble>
References: <20180115164428.498966552@infradead.org>
 <20180115164909.836510626@infradead.org>
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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