KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's not zero

A minor optimization to WRMSR MSR_IA32_RTIT_CTL when necessary.

Opportunistically refine the comment to call out that KVM requires
VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest.
Reviewed-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NXiaoyao Li <xiaoyao.li@intel.com>
Message-Id: <20210827070249.924633-2-xiaoyao.li@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1074,8 +1074,12 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
 		pt_load_msr(&vmx->pt_desc.host, vmx->pt_desc.addr_range);
 	}
 
-	/* Reload host state (IA32_RTIT_CTL will be cleared on VM exit). */
-	wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
+	/*
+	 * KVM requires VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest,
+	 * i.e. RTIT_CTL is always cleared on VM-Exit.  Restore it if necessary.
+	 */
+	if (vmx->pt_desc.host.ctl)
+		wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
 }
 
 void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,