x86/microcode: adjust sequence of controlling KVM guest and EPC

The current sequence to control SGX-enabled guest and EPC during
SGX SVN update is as follows:

lock EPC -> halt guest -> resume guest -> unlock EPC

There is time window that guest has chance to try to touch EPC while
EPC is "locked" for SVN update. To avoid any potential issue, change
the order as below, to ensure guest will absolutely have no chance
to do anything before SVN update completes.

halt guest -> lock EPC -> unlock EPC -> resume guest

Signed-off-by: Cathy Zhang <cathy.zhang@intel.com>

arch/x86/kernel/cpu/microcode/intel.c

@@ -1009,8 +1009,8 @@ int update_cpusvn_intel(void)
 {
 	int ret;
 
-	sgx_lock_epc();
 	sgx_kvm_notifier_halt();
+	sgx_lock_epc();
 	ret = sgx_zap_pages();
 	if (ret)
 		goto out;
@@ -1032,8 +1032,8 @@ int update_cpusvn_intel(void)
 	}
 
 out:
-	sgx_kvm_notifier_resume();
 	sgx_unlock_epc();
+	sgx_kvm_notifier_resume();
 
 	return ret;
 }