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

[Sean Christopherson]

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.
> 
> 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.
> 
> 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   |  4 +--
>  arch/x86/kvm/cpuid.h   |  2 ++
>  arch/x86/kvm/vmx/vmx.c | 65 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 69 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index dd387a785c1e..4166c4fcad1e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -371,13 +371,13 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry, int index)
>  		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ |
>  		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
>  		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> -		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B);
> +		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | F(SHSTK);
>  
>  	/* cpuid 7.0.edx*/
>  	const u32 kvm_cpuid_7_0_edx_x86_features =
>  		F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(SPEC_CTRL) |
>  		F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
> -		F(MD_CLEAR);
> +		F(MD_CLEAR) | F(IBT);

Advertising CET to userspace/guest needs to be done at the end of the
series, or at least after CR4.CET is no longer reserved, e.g. KVM_SET_SREGS
will fail and the guest will get a #GP when trying to set CR4.CET.

I'm pretty sure I've said this at least twice in previous versions of
this series...

>  
>  	/* cpuid 7.1.eax */
>  	const u32 kvm_cpuid_7_1_eax_x86_features =
> 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..db03d9dc1297 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2918,6 +2918,24 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  	vmcs_writel(GUEST_CR3, guest_cr3);
>  }
>  
> +static bool guest_cet_allowed(struct kvm_vcpu *vcpu, u32 feature, u32 mode)
> +{
> +	u64 kvm_xss = kvm_supported_xss();
> +
> +	/*
> +	 * Sanity check for guest CET dependencies, guest_cpu_has(SHSTK|IBT) has
> +	 * implied corresponding host CET status check.
> +	 */
> +	if (feature == X86_FEATURE_SHSTK)
> +		return guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) &&
> +		       (kvm_xss & mode);
> +	else if (feature == X86_FEATURE_IBT)
> +		return guest_cpuid_has(vcpu, X86_FEATURE_IBT) &&
> +		       (kvm_xss & mode);
> +
> +	return false;
> +}
> +
>  int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -7001,6 +7019,50 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>  
> +static void vmx_pass_cet_msrs(struct kvm_vcpu *vcpu)

"pass" isn't accurate, this function also does the opposite.  Maybe 
vmx_update_intercept_for_cet_msr()?  Or reuse the PT naming and go with
cet_update_intercept_for_msr()?

> +{
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	unsigned long *msr_bitmap = vmx->vmcs01.msr_bitmap;
> +
> +	/*
> +	 * U_CET is required for USER CET, per CET spec., meanwhile U_CET and
> +	 * PL3_SPP are a bundle for USER CET xsaves.
> +	 */
> +	if (guest_cet_allowed(vcpu, X86_FEATURE_SHSTK, XFEATURE_MASK_CET_USER) ||
> +	    guest_cet_allowed(vcpu, X86_FEATURE_IBT, XFEATURE_MASK_CET_USER)) {

IMO, the guest_cet_allowed() wrappers do more harm than good, e.g. I find
this easier to understand because it doesn't require digging into a random
helper.

	if ((kvm_supported_xss() & XFEATURE_MASK_CET_USER) &&
	    (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
	     guest_cpuid_has(vcpu, X86_FEATURE_IBT)))

> +		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);
> +	} else {
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_U_CET, MSR_TYPE_RW, true);
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL3_SSP, MSR_TYPE_RW, true);
> +	}
> +	/*
> +	 * S_CET is required for KERNEL CET, meanwhile PL0_SSP ... PL2_SSP are a bundle
> +	 * for CET KERNEL xsaves.
> +	 */
> +	if (guest_cet_allowed(vcpu, X86_FEATURE_SHSTK, XFEATURE_MASK_CET_KERNEL) ||
> +	    guest_cet_allowed(vcpu, X86_FEATURE_IBT, 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);
> +		else
> +			vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_INT_SSP_TAB,
> +						  MSR_TYPE_RW, true);
> +	} else {
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_S_CET, MSR_TYPE_RW, true);
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL0_SSP, MSR_TYPE_RW, true);
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL1_SSP, MSR_TYPE_RW, true);
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL2_SSP, MSR_TYPE_RW, true);
> +		vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, true);
> +	}
> +}
> +
>  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.

Allowing KVM_SET_CPUID{2} while running a nested guest seems bogus, can we
kill that path entirely with -EINVAL?

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