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;
