Re: [PATCH v3 20/21] KVM:x86: Enable kernel IBT support for guest

[Yang, Weijiang]

On 6/24/2023 8:03 AM, Sean Christopherson wrote:
> On Thu, May 11, 2023, Yang Weijiang wrote:
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index a2494156902d..1d0151f9e575 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -711,6 +711,7 @@ static bool is_valid_passthrough_msr(u32 msr)
> >   		return true;
> >   	case MSR_IA32_U_CET:
> >   	case MSR_IA32_PL3_SSP:
> > +	case MSR_IA32_S_CET:
> >   		return true;
> >   	}
> >   
> > @@ -2097,14 +2098,18 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   			msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
> >   		break;
> >   	case MSR_IA32_U_CET:
> > +	case MSR_IA32_S_CET:
> >   	case MSR_IA32_PL3_SSP:
> >   	case MSR_KVM_GUEST_SSP:
> >   		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> >   			return 1;
> > -		if (msr_info->index == MSR_KVM_GUEST_SSP)
> > +		if (msr_info->index == MSR_KVM_GUEST_SSP) {
> Unnecessary curly braces.

Something in my mind must be wrong :-), will remove them.

> >   			msr_info->data = vmcs_readl(GUEST_SSP);
> > -		else
> > +		} else if (msr_info->index == MSR_IA32_S_CET) {
> > +			msr_info->data = vmcs_readl(GUEST_S_CET);
> > +		} else {
> >   			kvm_get_xsave_msr(msr_info);
> > +		}
> >   		break;
> >   	case MSR_IA32_DEBUGCTLMSR:
> >   		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> > @@ -2419,6 +2424,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   			vmx->pt_desc.guest.addr_a[index / 2] = data;
> >   		break;
> >   	case MSR_IA32_U_CET:
> > +	case MSR_IA32_S_CET:
> >   	case MSR_IA32_PL3_SSP:
> >   	case MSR_KVM_GUEST_SSP:
> >   		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> > @@ -2430,10 +2436,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   		if ((msr_index == MSR_IA32_PL3_SSP ||
> >   		     msr_index == MSR_KVM_GUEST_SSP) && (data & GENMASK(2, 0)))
> >   			return 1;
> > -		if (msr_index == MSR_KVM_GUEST_SSP)
> > +		if (msr_index == MSR_KVM_GUEST_SSP) {
> >   			vmcs_writel(GUEST_SSP, data);
> > -		else
> > +		} else if (msr_index == MSR_IA32_S_CET) {
> > +			vmcs_writel(GUEST_S_CET, data);
> > +		} else {
> Same here.
>
> >   			kvm_set_xsave_msr(msr_info);
> > +		}
> >   		break;
> >   	case MSR_IA32_PERF_CAPABILITIES:
> >   		if (data && !vcpu_to_pmu(vcpu)->version)
> > @@ -7322,6 +7331,19 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >   
> >   	kvm_wait_lapic_expire(vcpu);
> >   
> > +	/*
> > +	 * Save host MSR_IA32_S_CET so that it can be reloaded at vm_exit.
> > +	 * No need to save the other two vmcs fields as supervisor SHSTK
> > +	 * are not enabled on Intel platform now.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_X86_KERNEL_IBT) &&
> > +	    (vm_exit_controls_get(vmx) & VM_EXIT_LOAD_CET_STATE)) {
> > +		u64 msr;
> > +
> > +		rdmsrl(MSR_IA32_S_CET, msr);
> Reading the MSR on every VM-Enter can't possibly be necessary.  At the absolute
> minimum, this could be moved outside of the fastpath; if the kernel modifies S_CET
> from NMI context, KVM is hosed.  And *if* S_CET isn't static post-boot, this can
> be done in .prepare_switch_to_guest() so long as S_CET isn't modified from IRQ
> context.

Agree with you.

> But unless mine eyes deceive me, S_CET is only truly modified during setup_cet(),
> i.e. is static post boot, which means it can be read once at KVM load time, e.g.
> just like host_efer.

I think handling S_CET like host_efer from usage perspective is possible 
given currently only

kernel IBT is enabled in kernel, I'll remove the code and initialize the 
vmcs field once like host_efer.

> The kernel does save/restore IBT when making BIOS calls, but if KVM is running a
> vCPU across a BIOS call then we've got bigger issues.

What's the problem you're referring to?

> > +		vmcs_writel(HOST_S_CET, msr);
> > +	}
> > +
> >   	/* The actual VMENTER/EXIT is in the .noinstr.text section. */
> >   	vmx_vcpu_enter_exit(vcpu, __vmx_vcpu_run_flags(vmx));
> >   
> > @@ -7735,6 +7757,13 @@ static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
> >   
> >   	incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> >   	vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, incpt);
> > +
> > +	/*
> > +	 * If IBT is available to guest, then passthrough S_CET MSR too since
> > +	 * kernel IBT is already in mainline kernel tree.
> > +	 */
> > +	incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT);
> > +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, incpt);
> >   }
> >   
> >   static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > @@ -7805,7 +7834,7 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> >   	/* Refresh #PF interception to account for MAXPHYADDR changes. */
> >   	vmx_update_exception_bitmap(vcpu);
> >   
> > -	if (kvm_cet_user_supported())
> > +	if (kvm_cet_user_supported() || kvm_cpu_cap_has(X86_FEATURE_IBT))
> Yeah, kvm_cet_user_supported() simply looks wrong.

These are preconditions to set up CET MSRs for guest, in 
vmx_update_intercept_for_cet_msr(),

the actual MSR control is based on guest_cpuid_has() results.