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Message-ID: <1516656469.9521.75.camel@infradead.org>
Subject: Re: [RFC 00/10] Speculation Control feature support
From:   David Woodhouse <dwmw2@infradead.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Mihai Carabas <mihai.carabas@oracle.com>
Cc:     linux-kernel@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, kvm@vger.kernel.org,
        x86@kernel.org
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         <20180121140205.GB21306@char.us.oracle.com>
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Date:   Mon, 22 Jan 2018 21:27:49 +0000
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On Sun, 2018-01-21 at 09:02 -0500, Konrad Rzeszutek Wilk wrote:
>=20
>=20
> Depend on what we expose to the guest. That is if the guest is not suppos=
e to have this exposed
> (say cpuid 27 bit is not exposed) then trap on the MSR (and give an #GP)?

I think for SPEC_CTRL we want to trap on the MSR anyway. Saving and
restoring is is *bizarrely* slow, apparently, even when it's zero.

I think we want to trap on the first access, and only then disable the
intercept and enable the save/restore. That way, sane guests that only
ever use retpoline and IBPB (which is write-only and doesn't need
saving) won't ever take the performance hit.

It's going to want this:=C2=A0https://patchwork.kernel.org/patch/10167667/

> Mihai (CC-ed) is working on this, when ready he can post an patch against=
 this tree?

That'd be useful; thanks. The latest (including the bits on top that we
probably aren't going to submit, with saner bits near the beginning)
should always be at
http://git.infradead.org/linux-retpoline.git/shortlog/refs/heads/ibpb


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