linux-kernel - Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Date:   Tue, 13 Feb 2018 08:15:08 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>, tglx@...utronix.de,
        x86@...nel.org, kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs



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, struct msr_data *msr_info)
> >  
> >  		vmx->spec_ctrl = data;
> >  
> > -		if (!data)
> > +		if (!data && !spectre_v2_ibrs_all())
> >  			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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