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Message-ID: <1520077075.7929.4.camel@amazon.co.uk>
Subject: Re: [PATCH 0/2] Backport IBPB on context switch to non-dumpable
 process
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
CC:     <stable@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Rik van Riel <riel@redhat.com>,
        "Ingo Molnar" <mingo@kernel.org>, <ak@linux.intel.com>,
        <karahmed@amazon.de>, <pbonzini@redhat.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
In-Reply-To: <20180303085454.GA23988@kroah.com>
References: <cover.1520026221.git.tim.c.chen@linux.intel.com>
         <20180303085454.GA23988@kroah.com>
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Content-Type: multipart/signed; micalg=sha-256; protocol="application/x-pkcs7-signature"; boundary="=-Z7kEP2drWRt5Nh7dR/J8"
Date:   Sat, 3 Mar 2018 12:37:55 +0100
MIME-Version: 1.0
From:   "Woodhouse, David" <dwmw@amazon.co.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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Content-Type: text/plain; charset="UTF-8"
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On Sat, 2018-03-03 at 09:54 +0100, Greg Kroah-Hartman wrote:
> On Fri, Mar 02, 2018 at 01:32:08PM -0800, Tim Chen wrote:
> >=20
> > Greg,
> >=20
> > I will like to propose backporting "x86/speculation: Use Indirect Branc=
h
> > Prediction Barrier on context switch" from commit 18bf3c3e in upstream
> > to 4.9 and 4.4 stable.=C2=A0=C2=A0The patch has already been ported to =
4.14 and
> > 4.15 stable.=C2=A0=C2=A0The patch needs mm context id that Andy added i=
n commit
> > f39681ed. I have lifted the mm context id change from Andy's upstream
> > patch and included it here.
>
> What does this patch "fix" in those older kernels?=C2=A0=C2=A0Is this a
> performance improvement or something else?

It's part of the Spectre variant 2 mitigation =E2=80=94 a full flush of the
branch prediction on context switch to a sensitive=C2=B9 process. It was th=
e
one I called out as "needs more attention" when I did the rest of the
retpoline etc backportingk, and Tim has now fixed it up. (Thanks).


=C2=B9 for now, "sensitive" means non-dumpable. This isn't perfect but it's
a reasonable approximation for now; it would be too expensive to do it
on *every* context switch. And for your purposes, the important part is
that it's what's upstream.
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