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Message-ID: <1518509708.12890.33.camel@infradead.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>, tglx@linutronix.de,
        x86@kernel.org, kvm@vger.kernel.org, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, arjan.van.de.ven@intel.com,
        dave.hansen@intel.com
In-Reply-To: <7e2e5ad1-49b6-1fdb-4a62-8ad6aefc30a0@redhat.com>
References: <1518449255-2182-1-git-send-email-dwmw@amazon.co.uk>
         <1518449255-2182-2-git-send-email-dwmw@amazon.co.uk>
         <7e2e5ad1-49b6-1fdb-4a62-8ad6aefc30a0@redhat.com>
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Date:   Tue, 13 Feb 2018 08:15:08 +0000
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On Tue, 2018-02-13 at 09:02 +0100, Paolo Bonzini wrote:
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -3419,13 +3419,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, s=
truct msr_data *msr_info)
> > =C2=A0
> > =C2=A0		vmx->spec_ctrl =3D data;
> > =C2=A0
> > -		if (!data)
> > +		if (!data && !spectre_v2_ibrs_all())
> > =C2=A0			break;
> This should check the value of IBRS_ALL in the VM, not in the host.

No, it's host we want. If IBRS_ALL is set in the host, we set the
actual hardware MSR once at boot time and never touch it again. The
SPEC_CTRL MSR we expose to guests is purely a no-op fiction.

If spectre_v2_ibrs_all() is true then KVM should *never* actually pass
through or touch the real MSR.

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