Date: Mon, 8 Jan 2018 13:47:07 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature
Message-ID: <20180108124707.GH32035@hirez.programming.kicks-ass.net>
References: <cover.1515204614.git.tim.c.chen@linux.intel.com>
 <b77c0cfc2d50ebdaa721f95012fd735cc374f04c.1515204614.git.tim.c.chen@linux.intel.com>
 <d7413d40-7581-a994-5185-9f11c014cb0f@intel.com>
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On Fri, Jan 05, 2018 at 07:12:12PM -0800, Dave Hansen wrote:
> On 01/05/2018 06:12 PM, Tim Chen wrote:
> >  .macro ENABLE_IBRS
> > -	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
> > +	testl	$1, dynamic_ibrs
> > +	jz	.Lskip_\@
> 
> There was an earlier suggestion to use STATIC_JUMP_IF_... here.  That's

Yes, and thanks for Cc'ing me... :/

> a good suggestion, but we encountered some issues with it either
> crashing the kernel at boot or not properly turning on/off.
> 
> We would love to do that minor really soon, but we figured everyone
> would rather see this version than wait for us to debug such a minor tweak.

Its not minor, you need that LFENCE without it. Which makes the whole
thing unconditionally expensive, which sucks.