linux-kernel - Re: [patch 21/24] x86/speculation: Prepare arch_smt

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Message-ID: <nycvar.YFH.7.76.1811230024140.21108@cbobk.fhfr.pm>
Date:   Fri, 23 Nov 2018 00:28:42 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 21/24] x86/speculation: Prepare arch_smt_update() for
 PRCTL mode

On Fri, 23 Nov 2018, Thomas Gleixner wrote:

> > So I'm wondering, shouldn't firmware_restrict_branch_speculation_start()/_end()
> > also enable/disable STIBP? It already enabled/disables IBRS.
> 
> IBRS includes STIBP. 

True.

> We don't use IBRS in the kernel otherwise because you'd have to do more 
> MSR writes on the protection boundaries.

Just for the record -- we do have an option for IBRS in our distro kernel 
on SKL+ systems.

There definitely is a measurable performance impact, but the MSR writes on 
protection boundaries are totally cheap compared to the actual IBRS 
runtime operation effect.

-- 
Jiri Kosina
SUSE Labs