linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Date:   Wed, 10 Jan 2018 15:56:24 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Kosina <jikos@...nel.org>,
        "asit.k.mallick" <asit.k.mallick@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, 2018-01-10 at 16:47 +0100, Andrea Arcangeli wrote:
> On Wed, Jan 10, 2018 at 03:24:17PM +0000, David Woodhouse wrote:
> > Since it achieves nothing¹ but to make userspace run slower, there's no
> > need to write it again on returning to userspace. It will perform that
> > function just fine without doing so.
> 
> Ok, very glad we are on the same page now.
> 
> Note that as far as I can tell there was no way to answer the above
> question by reading the spec.

The spec does, I concede, leave something to be desired. This sentence
in particular — and it really is a single sentence — caused me to throw
my toys out of the pram and demand verbal explanations (which is
perhaps the only reason I managed to work it out):

"If IBRS is set, near returns and near indirect jumps/calls will not allow their
speculative target address to be controlled by code that executed in a less
privileged prediction mode before the IBRS mode was last written with a
value of 1 or on another logical processor so long as all RSB entries from the
previous less privileged prediction mode are overwritten"

But it *does* have the words "before the IBRS mode was last written
with a value of 1" in there somewhere... ;)
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