linux-kernel - Re: [PATCH v7 3/7] KVM: VMX: Pass through CET related MSRs to Guest

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Date:   Thu, 17 Oct 2019 13:04:11 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     Jim Mattson <jmattson@...gle.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v7 3/7] KVM: VMX: Pass through CET related MSRs to Guest

On Wed, Oct 09, 2019 at 02:15:09PM +0800, Yang Weijiang wrote:
> On Wed, Oct 02, 2019 at 11:18:32AM -0700, Jim Mattson wrote:
> > > +       kvm_xss = kvm_supported_xss();
> > > +       cet_en = guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> > > +                guest_cpuid_has(vcpu, X86_FEATURE_IBT);
> > > +       /*
> > > +        * U_CET is a must for USER CET, per CET spec., U_CET and PL3_SPP are
> > > +        * a bundle for USER CET xsaves.
> > > +        */
> > > +       if (cet_en && (kvm_xss & XFEATURE_MASK_CET_USER)) {
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_U_CET, MSR_TYPE_RW);
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL3_SSP, MSR_TYPE_RW);
> > > +       }
> > 
> > Since this is called from vmx_cpuid_update, what happens if cet_en was
> > previously true and now it's false?
> > 
> Yes, it's likely, but guest CPUID usually is fixed before
> guest is launched, do you have any suggestion?

Re-enable interception.  kvm_x86_ops->cpuid_update() is only called in
the ioctl flow, i.e. it's not performance critical.

> > > +       /*
> > > +        * S_CET is a must for KERNEL CET, PL0_SSP ... PL2_SSP are a bundle
> > > +        * for CET KERNEL xsaves.
> > > +        */
> > > +       if (cet_en && (kvm_xss & XFEATURE_MASK_CET_KERNEL)) {
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_S_CET, MSR_TYPE_RW);
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL0_SSP, MSR_TYPE_RW);
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL1_SSP, MSR_TYPE_RW);
> > > +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL2_SSP, MSR_TYPE_RW);
> > > +
> > > +               /* SSP_TAB only available for KERNEL SHSTK.*/
> > > +               if (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
> > > +                       vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_INT_SSP_TAB,
> > > +                                                     MSR_TYPE_RW);
> > > +       }
> > > +}
> > > +
> > >  static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
> > >  {
> > >         struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > @@ -7025,6 +7062,8 @@ 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);
> > > +
> > > +       vmx_intercept_cet_msrs(vcpu);
> > >  }
> > >
> > >  static void vmx_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> > > --
> > > 2.17.2
> > >