linux-kernel - Re: [PATCH 20/24] objtool: Another static block fail

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Date:   Wed, 31 Jan 2018 10:07:06 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        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>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 20/24] objtool: Another static block fail



On Wed, 2018-01-31 at 11:01 +0100, Peter Zijlstra wrote:
> On Tue, Jan 30, 2018 at 09:12:21PM -0600, Josh Poimboeuf wrote:
> > 
> > Or, maybe we should just forget the whole thing and just stick with the
> > dynamic IBRS checks with lfence.  Yes, it's less ideal for the kernel,
> > but adding these acrobatics to objtool also has a cost.
> 
> For now, IBRS seems off the table entirely. But no, I really don't want
> to have to unconditionally eat the LFENCE cost in all those sites.

There's also alternatives. And without the IBRS-on-kernel-entry bits
there aren't that many call sites that really need this anyway and
don't have *other* conditionals that really are runtime-only (like
dumpable etc.).
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