linux-kernel - Re: [PATCH v8 3/7] KVM: VMX: Pass through CET related MSRs

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Date:   Wed, 18 Dec 2019 08:02:28 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com, yu-cheng.yu@...el.com
Subject: Re: [PATCH v8 3/7] KVM: VMX: Pass through CET related MSRs

On Wed, Dec 18, 2019 at 09:55:13PM +0800, Yang Weijiang wrote:
> On Tue, Dec 17, 2019 at 04:34:55PM -0800, Sean Christopherson wrote:
> > On Mon, Dec 16, 2019 at 10:18:16AM +0800, Yang Weijiang wrote:
> > > On Tue, Dec 10, 2019 at 01:18:21PM -0800, Sean Christopherson wrote:
> > > > On Fri, Nov 01, 2019 at 04:52:18PM +0800, Yang Weijiang wrote:
> > > > > CET MSRs pass through Guest directly to enhance performance.
> > > > > CET runtime control settings are stored in MSR_IA32_{U,S}_CET,
> > > > > Shadow Stack Pointer(SSP) are stored in MSR_IA32_PL{0,1,2,3}_SSP,
> > > > > SSP table base address is stored in MSR_IA32_INT_SSP_TAB,
> > > > > these MSRs are defined in kernel and re-used here.
> > > > > 
> > >  > +
> > > > >  static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
> > > > >  {
> > > > >  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > > > @@ -7025,6 +7087,9 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
> > > > >  	if (boot_cpu_has(X86_FEATURE_INTEL_PT) &&
> > > > >  			guest_cpuid_has(vcpu, X86_FEATURE_INTEL_PT))
> > > > >  		update_intel_pt_cfg(vcpu);
> > > > > +
> > > > > +	if (!is_guest_mode(vcpu))
> > > > > +		vmx_pass_cet_msrs(vcpu);
> > > > 
> > > > Hmm, this looks insufficent, e.g. deliberately toggling CET from on->off
> > > > while in guest mode would put KVM in a weird state as the msr bitmap for
> > > > L1 would still allow L1 to access the CET MSRs.
> > > >
> > > Hi, Sean,
> > > I don't get you, there's guest mode check before access CET msrs, it'll
> > > fail if it's in guest mode.
> > 
> > KVM can exit to userspae while L2 is active.  If userspace then did a
> > KVM_SET_CPUID2, e.g. instead of KVM_RUN, vmx_cpuid_update() would skip
> > vmx_pass_cet_msrs() and KVM would never update L1's MSR bitmaps.
> >
> Thanks, it makes sense to me. Given current implementation, how about
> removing above check and adding it in CET CPUID
> enumeration for L2 so that no CET msrs passed through to L2 when
> is_guest_mode() is true?

But you still need to update L1's MSR bitmaps.  That can obviously be done
all at once, but it's annoying and IMO unnecessarily complex.

> > > > Allowing KVM_SET_CPUID{2} while running a nested guest seems bogus, can we
> > > > kill that path entirely with -EINVAL?
> > > >
> > > Do you mean don't expose CET cpuids to L2 guest?
> > 
> > I mean completely disallow KVM_SET_CPUID and KVM_SET_CPUID2 if
> > is_guest_mode() is true.  My question is mostly directed at Paolo and
> > anyone else that has an opinion on whether we can massage the ABI to
> > retroactively change KVM_SET_CPUID{2} behavior.
>
> This sounds like something deserving an individual patch after get
> agreement in community. I'll put it aside right now.

Normally I would agree, but I think in this case it would significantly
reduce the complexity and implementation cost of CET support.  I'll send a
patch to kickstart the conversation, it's a tiny change in terms of code.