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Message-ID: <1517172296.6624.84.camel@infradead.org>
Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From:   David Woodhouse <dwmw2@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     KarimAllah Ahmed <karahmed@amazon.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Asit Mallick <asit.k.mallick@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>, daniel.kiper@oracle.com
In-Reply-To: <86F5D9C0-7B11-448D-954F-D2EF6CBE80EC@amacapital.net>
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         <4DCAF18F-C86A-4CBC-A9CC-CC01BF63313F@oracle.com>
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Date:   Sun, 28 Jan 2018 20:44:56 +0000
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On Sun, 2018-01-28 at 12:40 -0800, Andy Lutomirski wrote:
>=20
> Do you mean that the host would intercept the guest WRMSR and do
> WRMSR itself?=C2=A0 I would suggest that doing so is inconsistent with th=
e
> docs.=C2=A0 As specified, doing WRMSR to write 1 to IBRS does *not*
> protect the guest.

I believe it does. Guest kernel is protected from any guest userspace
predictions learned before IBRS was last set to 1 in *any* mode,
including host.

> For that matter, what are the semantics of VMRESUME doing a write to
> IBRS as part of its MSR switch?=C2=A0 Is it treated as IBRS=3D1 from gues=
t
> context?

Why does it matter? We *have* confirmed, FWIW, that VMRESUME writing 1
to IBRS as part of its MSR switch when it was already 1 is not
optimised away and *is* treated as writing IBRS=3D1 again.
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