[NTOS:KE/x64][SDK] Add HANDLE_USER_APCS asm macro

This also fixes delivering APCs from the system call handler, which previously would have clobbered rax. Also don't use the thread's TrapFrame member, which is not always set, when returning.

sdk/include/asm/trapamd64.inc

@@ -37,6 +37,19 @@ IrqlIsPassive:
 #endif
 ENDM
 
+// Checks for user APCs and delivers them if necessary.
+// Clobbers all volatile registers except rax.
+MACRO(HANDLE_USER_APCS, ThreadReg, TrapFrame)
+    LOCAL NoUserApcPending
+
+    /* Check for pending user APC */
+    cmp byte ptr [ThreadReg + ThApcState + AsUserApcPending], 0
+    jz NoUserApcPending
+    lea rcx, [TrapFrame]
+    call KiInitiateUserApc
+NoUserApcPending:
+ENDM
+
 APIC_EOI = HEX(0FFFFFFFFFFFE00B0)
 
 TF_VOLATILES            = HEX(01)
@@ -195,7 +208,6 @@ ENDM
  */
 MACRO(ExitTrap, Flags)
     LOCAL kernel_mode_return
-    LOCAL NoUserApc
 
     ASSERT_TRAP_FRAME_IRQL_VALID rbp
 
@@ -217,12 +229,8 @@ MACRO(ExitTrap, Flags)
     jz kernel_mode_return
 
     if (Flags AND TF_CHECKUSERAPC)
-        /* Load current thread into r10 */
         mov r10, gs:[PcCurrentThread]
-        cmp byte ptr [r10 + KTHREAD_UserApcPending], 0
-        je NoUserApc
-        call KiInitiateUserApc
-    NoUserApc:
+        HANDLE_USER_APCS r10, rbp
     endif
 
     ASSERT_TRAP_FRAME_INTS_ENABLED rbp