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Date: Mon, 15 Jan 2018 19:12:30 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>,
        linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        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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        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: <20180115181230.GD2228@hirez.programming.kicks-ass.net>
References: <20180115164428.498966552@infradead.org>
 <20180115164909.836510626@infradead.org>
 <20180115180405.cdzdgw6tocbsmbrf@treble>
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On Mon, Jan 15, 2018 at 12:04:05PM -0600, Josh Poimboeuf wrote:
> 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?

Can do, but adds permanent overhead for the fake table entries, also the
alternative in _static_cpu_has is slightly more complex, but it would
work I think.

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

Almost did that, but I figured I'd give this a try first. But yes I
agree it is somewhat ugly.