Re: [PATCH v2] KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02

[Paolo Bonzini]

On 04/05/20 14:01, Alexander Graf wrote:
> I like the WARN_ON :). It should be almost free during execution, but
> helps us catch problems early.

Yes, it's nice.  I didn't mind the "buddy" argument either, but if we're 
going to get a bool I prefer positive logic so I'd like to squash this:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b57420f3dd8f..299393750a18 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -304,7 +304,13 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct 
loaded_vmcs *vmcs)
        prev = vmx->loaded_vmcs;
        WARN_ON_ONCE(prev->cpu != cpu || prev->vmcs != per_cpu(current_vmcs, 
cpu));
        vmx->loaded_vmcs = vmcs;
-       vmx_vcpu_load_vmcs(vcpu, cpu, true);
+
+       /*
+        * This is the same guest from our point of view, so no
+        * indirect branch prediction barrier is needed.  The L1
+        * guest can protect itself with retpolines, IBPB or IBRS.
+        */
+       vmx_vcpu_load_vmcs(vcpu, cpu, false);
        vmx_sync_vmcs_host_state(vmx, prev);
        put_cpu();
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 669e14947ba9..0f9c8d2dd7f6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1311,7 +1311,7 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int 
cpu)
                pi_set_on(pi_desc);
 }
 
-void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_switch)
+void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool need_ibpb)
 {
        struct vcpu_vmx *vmx = to_vmx(vcpu);
        bool already_loaded = vmx->loaded_vmcs->cpu == cpu;
@@ -1336,7 +1336,7 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, 
bool nested_switch)
        if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
                per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
                vmcs_load(vmx->loaded_vmcs->vmcs);
-               if (!nested_switch)
+               if (need_ibpb)
                        indirect_branch_prediction_barrier();
        }
 
@@ -1378,7 +1378,7 @@ void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
        struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-       vmx_vcpu_load_vmcs(vcpu, cpu, false);
+       vmx_vcpu_load_vmcs(vcpu, cpu, true);
 
        vmx_vcpu_pi_load(vcpu, cpu);
 
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index fa61dc802183..e584ee9b3e94 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -320,7 +320,7 @@ struct kvm_vmx {
 };
 
 bool nested_vmx_allowed(struct kvm_vcpu *vcpu);
-void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_switch);
+void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool need_ibpb);
 void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
 int allocate_vpid(void);
 void free_vpid(int vpid);


Paolo