KVM: nVMX: Don't "put" vCPU or host state when switching VMCS

When switching between vmcs01 and vmcs02, KVM isn't actually switching
between guest and host.  If guest state is already loaded (the likely,
if not guaranteed, case), keep the guest state loaded and manually swap
the loaded_cpu_state pointer after propagating saved host state to the
new vmcs0{1,2}.

Avoiding the switch between guest and host reduces the latency of
switching between vmcs01 and vmcs02 by several hundred cycles, and
reduces the roundtrip time of a nested VM by upwards of 1000 cycles.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index a090bfb10796..856ac90ce10a 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -248,18 +248,39 @@ static void free_nested(struct kvm_vcpu *vcpu)
 	free_loaded_vmcs(&vmx->nested.vmcs02);
 }
 
+static void vmx_sync_vmcs_host_state(struct vcpu_vmx *vmx,
+				     struct loaded_vmcs *prev)
+{
+	struct vmcs_host_state *dest, *src;
+
+	if (unlikely(!vmx->guest_state_loaded))
+		return;
+
+	src = &prev->host_state;
+	dest = &vmx->loaded_vmcs->host_state;
+
+	vmx_set_host_fs_gs(dest, src->fs_sel, src->gs_sel, src->fs_base, src->gs_base);
+	dest->ldt_sel = src->ldt_sel;
+#ifdef CONFIG_X86_64
+	dest->ds_sel = src->ds_sel;
+	dest->es_sel = src->es_sel;
+#endif
+}
+
 static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	struct loaded_vmcs *prev;
 	int cpu;
 
 	if (vmx->loaded_vmcs == vmcs)
 		return;
 
 	cpu = get_cpu();
-	vmx_vcpu_put(vcpu);
+	prev = vmx->loaded_vmcs;
 	vmx->loaded_vmcs = vmcs;
 	vmx_vcpu_load(vcpu, cpu);
+	vmx_sync_vmcs_host_state(vmx, prev);
 	put_cpu();
 
 	vm_entry_controls_reset_shadow(vmx);