CPUID Faulting is a feature about CPUID instruction. When CPUID Faulting is enabled, all execution of the CPUID instruction outside system-management mode (SMM) cause a general-protection (#GP) if the CPL > 0. About this feature, detailed information can be found at https://www.intel.com/content/dam/www/public/us/en/documents/application-notes/virtualization-technology-flexmigration-application-note.pdf There is an issue that current kvm doesn't switch the value of MSR_MISC_FEATURES_ENABLES between host and guest. If MSR_MISC_FEATURES_ENABLES exists on the hardware cpu, and host enables CPUID faulting (setting the bit 0 of MSR_MISC_FEATURES_ENABLES), it will impact the guest's behavior because cpuid faulting is enabled by host and passed to guest. >From my tests, when host enables cpuid faulting, it causes guest boot failure when guest uses *modprobe* to load modules. Below is the error log: [ 1.233556] traps: modprobe[71] general protection fault ip:7f0077f6495c sp:7ffda148d808 error:0 in ld-2.17.so[7f0077f4d000+22000] [ 1.237780] traps: modprobe[73] general protection fault ip:7fad5aba095c sp:7ffd36067378 error:0 in ld-2.17.so[7fad5ab89000+22000] [ 1.241930] traps: modprobe[75] general protection fault ip:7f3edb89495c sp:7fffa1a81308 error:0 in ld-2.17.so[7f3edb87d000+22000] [ 1.245998] traps: modprobe[77] general protection fault ip:7f91d670895c sp:7ffc25fa7f38 error:0 in ld-2.17.so[7f91d66f1000+22000] [ 1.250016] traps: modprobe[79] general protection fault ip:7f0ddbbdc95c sp:7ffe9c34f8d8 error:0 in ld-2.17.so[7f0ddbbc5000+22000] *modprobe* calls CPUID instruction thus causing cpuid faulting in guest. At the end, because guest cannot *modprobe* modules, it boots failure. This patch switches MSR_MISC_FEATURES_ENABLES between host and guest when hardware has this MSR. This patch doesn't confict with the commit db2336a80489 ("KVM: x86: virtualize cpuid faulting"), which provides a software emulation of cpuid faulting for x86 arch. Below analysing how cpuid faulting will work after applying this patch: 1. If host cpu is AMD. It doesn't have MSR_MISC_FEATURES_ENABLES, so we can just use the software emulation of cpuid faulting. 2. If host cpu is Intel and it doesn't have MSR_MISC_FEATURES_ENABLES. The same as case 1, we can just use the software emulation of cpuid faulting. 3. If host cpu is Intel and it has MSR_MISC_FEATURES_ENABLES. With this patch, it will write guest's value into MSR_MISC_FEATURES_ENABLES when vm entry. If guest enables cpuid faulting and when guest calls CPUID instruction with CPL > 0, it will cause #GP exception in guest instead of VM exit because of CPUID, thus it doesn't go to the kvm emualtion path but ues the hardware feature. Also it's a benefit that we needn't use VM exit to inject #GP to emulate cpuid faulting feature. Intel SDM vol3.25.1.1 specifies the priority between cpuid faulting and CPUID instruction. Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx> --- arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 30a6bcd735ec..90707fae688e 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6321,6 +6321,23 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx) msrs[i].host, false); } +static void atomic_switch_msr_misc_features_enables(struct kvm_vcpu *vcpu) +{ + u64 host_msr; + struct vcpu_vmx *vmx = to_vmx(vcpu); + + /* if MSR MISC_FEATURES_ENABLES doesn't exist on the hardware, do nothing*/ + if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &host_msr)) + return; + + if (host_msr == vcpu->arch.msr_misc_features_enables) + clear_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES); + else + add_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES, + vcpu->arch.msr_misc_features_enables, + host_msr, false); +} + static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val) { vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val); @@ -6562,6 +6579,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) atomic_switch_perf_msrs(vmx); + atomic_switch_msr_misc_features_enables(vcpu); + vmx_update_hv_timer(vcpu); /* -- 2.19.1
