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

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Date:   Mon, 16 Dec 2019 10:18:16 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, 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 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.

> 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?
thanks!

> >  }
> >  
> >  static void vmx_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> > -- 
> > 2.17.2
> >