ptos/amd64: ps: Add support for CPL 3 processes

Signed-off-by: Chloe M <chloe@mensia.org>

service/ptos/arch/amd64/ps/context.c

@@ -20,8 +20,12 @@ HalContextSwitch(TRAP_FRAME *Frame)
     PROCESS *NextParent;
     TCB *ThisTcb, *NextTcb;
     KPCR *Kpcr;
+    MCB *Mcb;
+    TSS_ENTRY *Tss;
+    UPTR KStackTop;
 
     Kpcr = HalKpcrCurrent();
+    Mcb = &Kpcr->Mcb;
     ThisThread = Kpcr->CurrentThread;
     NextThread = NULL;
 
@@ -54,6 +58,12 @@ HalContextSwitch(TRAP_FRAME *Frame)
     NextParent = NextThread->Parent;
     HalMmuWriteVas(&NextParent->Vas);
 
+    /* Update the task state segment with kstack */
+    KStackTop = NextTcb->KStackBase + NextTcb->KStackLength;
+    Tss = Mcb->Tss;
+    Tss->Rsp0Low = KStackTop & 0xFFFFFFFF;
+    Tss->Rsp0High = (KStackTop >> 32) & 0xFFFFFFFF;
+
     /* Context switch here */
     Kpcr->CurrentThread = NextThread;
     NextTcb = &NextThread->Tcb;