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sgi: preserve R27 in setregisters(), use setregisters() in noted()

GEVector() saves the exception return PC in Ureg.r27 which needs
to be preserved.

there should be no reason for the user to change the status
register from noted() eigther, so we now just use setregisters()
in noted() to restore previous general purpose registers. this
means that CU1 will always be off after noted() because notify()
has disabled the FPU on entry and set fpstatus to FPinactive
if it was on. once user starts using FPU again, it will trap and
restore fpu registers.

--- a/sys/src/9/sgi/trap.c

+++ b/sys/src/9/sgi/trap.c

@@ -576,22 +576,6 @@

 }

 /*

- * Check that status is OK to return from note.

- */

-int

-validstatus(ulong kstatus, ulong ustatus)

-{

-//	if((kstatus & (INTMASK|KX|SX|UX)) != (ustatus & (INTMASK|KX|SX|UX)))

-	if((kstatus & INTMASK) != (ustatus & INTMASK))

-		return 0;

-	if((ustatus&(KSU|ERL|EXL|IE)) != (KUSER|EXL|IE))

-		return 0;

-	if(ustatus & (0xFFFF0000&~CU1))	/* no CU3, CU2, CU0, RP, FR, RE, DS */

-		return 0;

-	return 1;

-}

-

-/*

  * Return user to state before notify(); called from user's handler.

  */

 void

@@ -613,20 +597,13 @@

 	nur = up->ureg;

 	oureg = (ulong)nur;

-	if((oureg & (BY2WD-1))

-	|| !okaddr((ulong)oureg-BY2WD, BY2WD+sizeof(Ureg), 0)){

+	if((oureg & (BY2WD-1)) || !okaddr((ulong)oureg-BY2WD, BY2WD+sizeof(Ureg), 0)){

 		pprint("bad up->ureg in noted or call to noted() when not notified\n");

 		qunlock(&up->debug);

 		pexit("Suicide", 0);

 	}

-	if(0 && !validstatus(kur->status, nur->status)) {

-		qunlock(&up->debug);

-		pprint("bad noted ureg status %#lux\n", nur->status);

-		pexit("Suicide", 0);

-	}

-

-	memmove(kur, up->ureg, sizeof(Ureg));

+	setregisters(kur, (char*)kur, (char*)up->ureg, sizeof(Ureg));

 	switch(arg0) {

 	case NCONT:

 	case NRSTR:				/* only used by APE */

@@ -873,10 +850,12 @@

 void

 setregisters(Ureg *xp, char *pureg, char *uva, int n)

 {

-	ulong status;

+	ulong status, r27;

+	r27 = xp->r27;			/* return PC for GEVector() */

 	status = xp->status;

 	memmove(pureg, uva, n);

+	xp->r27 = r27;

 	xp->status = status;

 }

-- 

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