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

[Yang Weijiang]

On Wed, Oct 02, 2019 at 11:18:32AM -0700, Jim Mattson wrote:
> On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> 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.
> 
> All of these new guest MSRs will have to be enumerated by
> KVM_GET_MSR_INDEX_LIST.
> 
Since CET feature is Intel platform specific, but looks like KVM_GET_MSR_INDEX_LIST
fetchs x86 common MSRs, I have patch in QEMU to support CET
MSRs, the patch is here:
https://patchwork.ozlabs.org/patch/1058265/

> > MSR_IA32_U_CET and MSR_IA32_PL3_SSP are used for user mode protection,
> > the contents could differ from process to process, therefore,
> > kernel needs to save/restore them during context switch, it makes
> > sense to pass through them so that the guest kernel can
> > use xsaves/xrstors to operate them efficiently. Other MSRs are used
> > for non-user mode protection. See CET spec for detailed info.
> 
> I assume that XSAVES & XRSTORS bypass the MSR permission bitmap, like
> other instructions that manipulate MSRs (e.g. SWAPGS, RDTSCP, etc.).
> Is the guest OS likely to use RDMSR/WRMSR to access these MSRs?
> 
Yes, exactly, you may check the CET kernel code.

> > The difference between CET VMCS state fields and xsave components is that,
> > the former used for CET state storage during VMEnter/VMExit,
> > whereas the latter used for state retention between Guest task/process
> > switch.
> >
> > Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/cpuid.c   |  1 +
> >  arch/x86/kvm/cpuid.h   |  2 ++
> >  arch/x86/kvm/vmx/vmx.c | 39 +++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 42 insertions(+)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 1aa86b87b6ab..0a47b9e565be 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -66,6 +66,7 @@ u64 kvm_supported_xss(void)
> >  {
> >         return KVM_SUPPORTED_XSS & kvm_x86_ops->supported_xss();
> >  }
> > +EXPORT_SYMBOL_GPL(kvm_supported_xss);
> >
> >  #define F(x) bit(X86_FEATURE_##x)
> >
> > diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> > index d78a61408243..1d77b880084d 100644
> > --- a/arch/x86/kvm/cpuid.h
> > +++ b/arch/x86/kvm/cpuid.h
> > @@ -27,6 +27,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> >
> >  int cpuid_query_maxphyaddr(struct kvm_vcpu *vcpu);
> >
> > +u64 kvm_supported_xss(void);
> > +
> >  static inline int cpuid_maxphyaddr(struct kvm_vcpu *vcpu)
> >  {
> >         return vcpu->arch.maxphyaddr;
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index a84198cff397..f720baa7a9ba 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7001,6 +7001,43 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
> >                 vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
> >  }
> >
> > +static void vmx_intercept_cet_msrs(struct kvm_vcpu *vcpu)
> 
> Nit: It seems like this function adjusts the MSR permission bitmap so
> as *not* to intercept the CET MSRs.
>
OK, will rename it.
> > +{
> > +       struct vcpu_vmx *vmx = to_vmx(vcpu);
> > +       unsigned long *msr_bitmap;
> > +       u64 kvm_xss;
> > +       bool cet_en;
> > +
> > +       msr_bitmap = vmx->vmcs01.msr_bitmap;
> 
> What about nested guests? (i.e. vmcs02).
> 
Hmm, I need to check the nested case, thank you.

> > +       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?
> > +       /*
> > +        * 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
> >