linux-kernel - Re: [PATCH 3/5] x86/ibrs: Add direct access support for MSR_IA32_SPEC

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Date:   Mon, 15 Jan 2018 15:45:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        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>
Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for
 MSR_IA32_SPEC_CTRL

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.