linux-kernel - Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

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Message-ID: <20180110182856.GQ6176@hirez.programming.kicks-ass.net>
Date:   Wed, 10 Jan 2018 19:28:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

On Wed, Jan 10, 2018 at 10:16:20AM -0800, Tim Chen wrote:
> On 01/10/2018 02:04 AM, Peter Zijlstra wrote:

> > What this Changelog fails to address is _WHY_ we need this. What does
> > this provide that retpoline does not.
> > 
> 
> Ok. I mentioned that in the cover letter that IBRS is a maximum security
> mode in the CPU itself to directly restrict all indirect branches to prevent SPECTRE v2.
> 
> I'll also include such comments in the commit log here.

That still doesn't say anything useful. Why and where is it better than
retpoline? Why would I ever want to use IBRS? Those are not questions
that have clear answers here.

>From what I can gather of the discussion earlier today is that pre SKL
IBRS is no better than retpoline and a whole lot slower.

On SKL+ retpoline is mostly there, but has a few dinky holes in and it
_might_ make sense to use IBRS.

But I feel it needs explaining what the exact holes are (pjt and dwmw2
had a fair enumeration IIRC) such that people can judge the risk.

No wishy washy maybe nonsense, clear language.