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 18:44:47 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        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

> > 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.

your question was specific to RSB not BTB. But please show the empirical evidence for RSB ?