linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Fri, 26 Jan 2018 18:43:06 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Laura Abbott <labbott@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Borislav Petkov <bp@...e.de>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        Radim Krcmár <rkrcmar@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Peter Anvin <hpa@...or.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On Fri, 2018-01-26 at 18:28 +0000, Van De Ven, Arjan wrote:
> > As you know well, I mean "we think Intel's document is not
> > correct".
>
> you asked before and even before you sent the email I confirmed to
> you that the document is correct
> 
> I'm not sure what the point is to then question that again 15 minutes
> later other than creating more noise.

Apologies, I hadn't seen the comment on IRC.

Sometimes the docs *don't* get it right, especially when they're
released in a hurry as that one was. I note there's a *fourth* version
of microcode-update-guidance.pdf available now, for example :)

So it is useful that you have explicitly stated that for *this*
specific concern, the document is in fact correct that SMEP saves us
from BTB and RSB pollution, *despite* the empirical evidence that those
structures only hold the low 31 bits.

I'm going to get back to other things now, although I'm sure others may
be very interested to reconcile the empirical evidence with what you
say, and want to know *how* that can be the case. Which I'm sure you
won't be able to say in public anyway.
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