linux-kernel - Re: [PATCH v2 07/10] x86/jump_label: Implement arch_static

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Date:   Thu, 18 Jan 2018 16:31:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        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 v2 07/10] x86/jump_label: Implement arch_static_assert()

On Thu, Jan 18, 2018 at 02:33:22PM +0100, Borislav Petkov wrote:
> On Tue, Jan 16, 2018 at 03:28:32PM +0100, Peter Zijlstra wrote:
> > Implement the static (branch) assertion. It simply emits the address
> > of the next instruction into a special section which objtool will read
> > and validate against either __jump_table or .altinstructions.
> > 
> > Use like:
> > 
> > 	if (static_branch_likely(_key)) {
> > 		arch_static_assert();
> > 		/* do stuff */
> > 	}
> > 
> > Or
> > 
> > 	if (static_cpu_has(_feat)) {
> > 		arch_static_assert();
> 
> It looks to me like the asserts can be moved into the macros and the
> assertion checking can happen by default ...?

Right until the point where someone writes:

	if (static_cpu_has(_feat) || ponies)

at which point it, rightfully, unconditionally asserts.

Not sure someone does that with static_cpu_has(), but there's
static_branch*() users that do exactly that.