linux-kernel - Re: [PATCH v9 3/7] KVM: VMX: Pass through CET related MSRs

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Date:   Wed, 4 Mar 2020 16:46:07 +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
Subject: Re: [PATCH v9 3/7] KVM: VMX: Pass through CET related MSRs

On Tue, Mar 03, 2020 at 01:51:53PM -0800, Sean Christopherson wrote:
> On Fri, Dec 27, 2019 at 10:11:29AM +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@...ux.intel.com>
> > Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> > Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> > ---
> >  arch/x86/kvm/cpuid.h   |  2 ++
> >  arch/x86/kvm/vmx/vmx.c | 48 ++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 50 insertions(+)
> > 
> > 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 477173e4a85d..61fc846c7ef3 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7091,6 +7091,52 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
> >  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
> >  }
> >  
> > +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
> > +{
> > +	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 ((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);
> > +	}
> 
> I prefer the style of pt_update_intercept_for_msr(), e.g.
> 
> 	flag = (kvm_supported_xss() & XFEATURE_MASK_CET_USER) &&
> 		(guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> 		 guest_cpuid_has(vcpu, X86_FEATURE_IBT));
> 
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_U_CET, MSR_TYPE_RW, flag);
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL3_SSP, MSR_TYPE_RW, flag);
> 
> 
> 	flag = (kvm_supported_xss() & XFEATURE_MASK_CET_KERNEL) &&
> 		(guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> 		 guest_cpuid_has(vcpu, X86_FEATURE_IBT));
> 
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_S_CET, MSR_TYPE_RW, flag);
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL0_SSP, MSR_TYPE_RW, flag);
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL1_SSP, MSR_TYPE_RW, flag);
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_PL2_SSP, MSR_TYPE_RW, flag);
> 
> 	/* SSP_TAB only available for KERNEL SHSTK.*/
> 	flag &= guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> 	vmx_set_intercept_for_msr(msr_bitmap, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, flag);
> 
>
Sure, will change that, thank you!

> > +	/*
> > +	 * S_CET is required for KERNEL CET, meanwhile PL0_SSP ... PL2_SSP are a bundle
> > +	 * for CET KERNEL xsaves.
> > +	 */
> > +	if ((kvm_supported_xss() & XFEATURE_MASK_CET_KERNEL) &&
> > +	    (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> > +	    guest_cpuid_has(vcpu, X86_FEATURE_IBT))) {
> > +		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);
> > @@ -7115,6 +7161,8 @@ 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_update_intercept_for_cet_msr(vcpu);
> >  }
> >  
> >  static void vmx_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> > -- 
> > 2.17.2
> >