On Thu, Oct 26, 2023, Sean Christopherson wrote: > On Wed, Oct 25, 2023, Pawan Gupta wrote: > > vmx_disable_fb_clear(vmx); > > LOL, nice. IIUC, setting FB_CLEAR_DIS is mutually exclusive with doing a late > VERW, as KVM will never set FB_CLEAR_DIS if the CPU is susceptible to X86_BUG_MDS. > But the checks aren't identical, which makes this _look_ sketchy. > > Can you do something like this to ensure we don't accidentally neuter the late VERW? > > static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) > { > vmx->disable_fb_clear = (host_arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) && > !boot_cpu_has_bug(X86_BUG_MDS) && > !boot_cpu_has_bug(X86_BUG_TAA); > > if (vmx->disable_fb_clear && > WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))) > vmx->disable_fb_clear = false; > > ... > } Alternatively, and maybe even preferably, this would make it more obvious that the two are mutually exclusive and would also be a (very, very) small perf win when the mitigation is enabled. diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 0936516cb93b..592103df1754 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -7236,7 +7236,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu, kvm_arch_has_assigned_device(vcpu->kvm)) mds_clear_cpu_buffers(); - vmx_disable_fb_clear(vmx); + if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF)) + vmx_disable_fb_clear(vmx); if (vcpu->arch.cr2 != native_read_cr2()) native_write_cr2(vcpu->arch.cr2); @@ -7249,7 +7250,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu, vmx->idt_vectoring_info = 0; - vmx_enable_fb_clear(vmx); + if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF)) + vmx_enable_fb_clear(vmx); if (unlikely(vmx->fail)) { vmx->exit_reason.full = 0xdead;