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Message-ID: <1516992793.30244.280.camel@infradead.org>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
From:   David Woodhouse <dwmw2@infradead.org>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Hansen, Dave" <dave.hansen@intel.com>,
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        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
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        "Mallick, Asit K" <asit.k.mallick@intel.com>,
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        KarimAllah Ahmed <karahmed@amazon.de>,
        Peter Anvin <hpa@zytor.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Joerg Roedel <joro@8bytes.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
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Date:   Fri, 26 Jan 2018 18:53:13 +0000
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On Fri, 2018-01-26 at 18:44 +0000, Van De Ven, Arjan wrote:
> your question was specific to RSB not BTB. But please show the empirical =
evidence for RSB ?

We were hypothesising, which should have been clear from:

On Fri, 2018-01-26 at 09:11 +0000, David Woodhouse wrote:
> Likewise if the RSB only stores the low 31 bits of the target, SMEP
> isn't much help there either.
>=C2=A0
> Do we need to look again at the fact that we've disabled the RSB-
> stuffing for SMEP?

... and later...=C2=A0

On Fri, 2018-01-26 at 17:31 +0000, David Woodhouse wrote:
> Note, we've switched from talking about BTB to RSB here, so this is a
> valid concern if the *RSB* only has the low bits of the target.

I'm glad to hear that it *isn't* a valid concern for the RSB and the
code in Linus' tree is correct.

Thank you for clearing that up.

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