Re: [PATCH 2/4] objtool: Implement jump_assert for _static_cpu_has()

[Peter Zijlstra <peterz@infradead.org>]

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.