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

Sean Christopherson
2023-06-24T00:03:10Z

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.

> 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.

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.

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.

> + 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.