KVM: vmx: skip VMWRITE of HOST_{FS,GS}_SEL when possible

On a 64-bit host, FS.sel and GS.sel are all but guaranteed to be 0,
which in turn means they'll rarely change.  Skip the VMWRITE for the
associated VMCS fields when loading host state if the selector hasn't
changed since the last VMWRITE.
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -2717,16 +2717,20 @@ static void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	gs_base = segment_base(gs_sel);
 #endif
 
-	host_state->fs_sel = fs_sel;
+	if (unlikely(fs_sel != host_state->fs_sel)) {
 		if (!(fs_sel & 7))
 			vmcs_write16(HOST_FS_SELECTOR, fs_sel);
 		else
 			vmcs_write16(HOST_FS_SELECTOR, 0);
-	host_state->gs_sel = gs_sel;
+		host_state->fs_sel = fs_sel;
+	}
+	if (unlikely(gs_sel != host_state->gs_sel)) {
 		if (!(gs_sel & 7))
 			vmcs_write16(HOST_GS_SELECTOR, gs_sel);
 		else
 			vmcs_write16(HOST_GS_SELECTOR, 0);
+		host_state->gs_sel = gs_sel;
+	}
 
 	vmcs_writel(HOST_FS_BASE, fs_base);
 	vmcs_writel(HOST_GS_BASE, gs_base);