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pc64: dont save/restore DS/ES/FS/GS segment registers on syscall or interrupt, they are ignored in long mode.

we do not support 32 bit processes and DS, ES, FS and GS segment
registers are ignored in long mode, so theres no point in saving
and restoring them.

--- a/sys/src/9/pc64/l.s

+++ b/sys/src/9/pc64/l.s

@@ -687,14 +687,6 @@

 	CLI

 	SWAPGS

-	MOVW	$UDSEL, AX

-	MOVW	AX, DS

-	MOVW	AX, ES

-

-	MOVW	$NULLSEL, AX

-	MOVW	AX, FS

-	MOVW	AX, GS

-

 	MOVL	$0, RMACH

 	MOVL	$0, RUSER

@@ -719,14 +711,8 @@

 	PUSHQ	R11				/* old flags */

 	PUSHQ	$UESEL				/* old code segment */

 	PUSHQ	CX				/* old ip */

-

 	SUBQ	$(16+16*8), SP			/* unsaved registers */

-	MOVW	$UDSEL, (15*8+0)(SP)

-	MOVW	ES, (15*8+2)(SP)

-	MOVW	FS, (15*8+4)(SP)

-	MOVW	GS, (15*8+6)(SP)

-

 	MOVQ	RMACH, (14*8)(SP)

 	MOVQ	RUSER, (13*8)(SP)

@@ -746,11 +732,6 @@

 	CLI

 	SWAPGS

-	MOVW	22(SP), GS

-	MOVW	20(SP), FS

-	MOVW	18(SP), ES

-	MOVW	16(SP), DS

-

 	MOVQ	8(SP), RMACH

 	MOVQ	0(SP), RUSER

@@ -785,10 +766,6 @@

 	MOVQ	RUSER, 0(SP)

 	MOVQ	RMACH, 8(SP)

-	MOVW	DS, 16(SP)

-	MOVW	ES, 18(SP)

-	MOVW	FS, 20(SP)

-	MOVW	GS, 22(SP)

 	SWAPGS

 	BYTE $0x65; MOVQ 0, RMACH		/* m-> (MOVQ GS:0x0, R15) */

@@ -834,10 +811,7 @@

 	JEQ	_iretnested

 	SWAPGS

-	MOVW	22(SP), GS

-	MOVW	20(SP), FS

-	MOVW	18(SP), ES

-	MOVW	16(SP), DS

+

 	MOVQ	8(SP), RMACH

 	MOVQ	0(SP), RUSER

--- a/sys/src/9/pc64/trap.c

+++ b/sys/src/9/pc64/trap.c

@@ -476,21 +476,19 @@

 	else

 		iprint("cpu%d: registers for kernel\n", m->machno);

-	iprint("  DS %.4uX      AX %.16lluX  BX %.16lluX  CX %.16lluX\n",

-		ureg->ds, ureg->ax, ureg->bx, ureg->cx);

-	iprint("  ES %.4uX      DX %.16lluX  SI %.16lluX  DI %.16lluX\n",

-		ureg->es, ureg->dx, ureg->si, ureg->di);

-	iprint("  FS %.4uX      BP %.16lluX  R8 %.16lluX  R9 %.16lluX\n",

-		ureg->fs, ureg->bp, ureg->r8, ureg->r9);

-	iprint("  GS %.4uX     R10 %.16lluX R11 %.16lluX R12 %.16lluX\n",

-		ureg->gs, ureg->r10, ureg->r11, ureg->r12);

-

-	iprint("  SS %.4lluX     R13 %.16lluX R14 %.16lluX R15 %.16lluX\n",

-		ureg->ss & 0xffff, ureg->r13, ureg->r14, ureg->r15);

-	iprint("  CS %.4lluX      PC %.16lluX  SP %.16lluX\n",

-		ureg->cs & 0xffff, ureg->pc, ureg->sp);

-

-	iprint("TYPE %.2lluX     ERROR %.4lluX           FLAGS %.8lluX\n",

+	iprint("  AX %.16lluX  BX %.16lluX  CX %.16lluX\n",

+		ureg->ax, ureg->bx, ureg->cx);

+	iprint("  DX %.16lluX  SI %.16lluX  DI %.16lluX\n",

+		ureg->dx, ureg->si, ureg->di);

+	iprint("  BP %.16lluX  R8 %.16lluX  R9 %.16lluX\n",

+		ureg->bp, ureg->r8, ureg->r9);

+	iprint(" R10 %.16lluX R11 %.16lluX R12 %.16lluX\n",

+		ureg->r10, ureg->r11, ureg->r12);

+	iprint(" R13 %.16lluX R14 %.16lluX R15 %.16lluX\n",

+		ureg->r13, ureg->r14, ureg->r15);

+	iprint("  CS %.4lluX   SS %.4lluX    PC %.16lluX  SP %.16lluX\n",

+		ureg->cs & 0xffff, ureg->ss & 0xffff, ureg->pc, ureg->sp);

+	iprint("TYPE %.2lluX  ERROR %.4lluX FLAGS %.8lluX\n",

 		ureg->type & 0xff, ureg->error & 0xffff, ureg->flags & 0xffffffff);

 	/*

@@ -507,7 +505,7 @@

 		if(ureg->type == 18)

 			dumpmcregs();

 	}

-	iprint("\n  ur %#p up %#p\n", ureg, up);

+	iprint("  ur %#p up %#p\n", ureg, up);

 }

@@ -854,7 +852,7 @@

 	ureg->pc = (uintptr)up->notify;

 	ureg->bp = (uintptr)up->ureg;		/* arg1 passed in RARG */

 	ureg->cs = UESEL;

-	ureg->ss = ureg->ds = ureg->es = UDSEL;

+	ureg->ss = UDSEL;

 	up->notified = 1;

 	up->nnote--;

 	memmove(&up->lastnote, &up->note[0], sizeof(Note));

@@ -952,8 +950,7 @@

 	ureg->sp = (uintptr)sp;

 	ureg->pc = entry;

 	ureg->cs = UESEL;

-	ureg->ss = ureg->ds = ureg->es = UDSEL;

-	ureg->fs = ureg->gs = NULLSEL;

+	ureg->ss = UDSEL;

 	ureg->r14 = ureg->r15 = 0;	/* extern user registers */

 	return (uintptr)USTKTOP-sizeof(Tos);		/* address of kernel/user shared data */

 }

@@ -981,11 +978,7 @@

 	flags = ureg->flags;

 	memmove(pureg, uva, n);

 	ureg->cs = UESEL;

-	ureg->ss = ureg->ds = ureg->es = UDSEL;

-	if(ureg->fs != UDSEL)

-		ureg->fs = NULLSEL;

-	if(ureg->gs != UDSEL)

-		ureg->gs = NULLSEL;

+	ureg->ss = UDSEL;

 	ureg->flags = (ureg->flags & 0x00ff) | (flags & 0xff00);

 	ureg->pc &= UADDRMASK;

 }