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Date: Mon, 15 Jan 2018 15:45:39 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
        Ashok Raj <ashok.raj@intel.com>, linux-kernel@vger.kernel.org,
        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>,
        Andrea Arcangeli <aarcange@redhat.com>,
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        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        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>
Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for
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On Mon, Jan 15, 2018 at 02:59:22PM +0100, David Woodhouse wrote:
> #define IF_FEATURE(ftr) if (static_cpu_has(ftr)) arch_static_assert,?
> 
> ? ?IF_FEATURE(key) {
> ? ? ? ?stuff();
> ? ?}
> 
> There might not be a sane way to do that though. And it's OK to have to
> manually annotate the call sites where this is for correctness and not
> purely optimisation.

Something like:

#define if_static_likely(_key) \
	if (static_branch_likely(_key) && (arch_static_assert(), true))

should work I think. The thing about static_cpu_has() is that it doesn't
use jump_labels but alternatives. I could of course also construct an
assert for that, but it needs different things.