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Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <1517172296.6624.84.camel@infradead.org>
Date:   Sun, 28 Jan 2018 12:53:15 -0800
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        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
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To:     David Woodhouse <dwmw2@infradead.org>
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> On Jan 28, 2018, at 12:44 PM, David Woodhouse <dwmw2@infradead.org> wrote:=

>=20
>> 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?  I would suggest that doing so is inconsistent with the
>> docs.  As specified, doing WRMSR to write 1 to IBRS does *not*
>> protect the guest.
>=20
> 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.

Hmm, you're probably right.

I would love to know what awful hack Intel did that resulted in these semant=
ics.

>=20
>> For that matter, what are the semantics of VMRESUME doing a write to
>> IBRS as part of its MSR switch?  Is it treated as IBRS=3D1 from guest
>> context?
>=20
> 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.

That's good news.=