Re: [PATCH v3 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

[Sean Christopherson <seanjc@google.com>]

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;