powerpc: Cleanup handling of the DSCR bit in the FSCR register

As suggested by paulus we can simplify the Data Stream Control Register
(DSCR) Facility Status and Control Register (FSCR) handling.

Firstly, we simplify the asm by using a rldimi.

Secondly, we now use the FSCR only to control the DSCR facility, rather
than both the FSCR and HFSCR.  Users will see no functional change from
this but will get a minor speedup as they will trap into the kernel only
once (rather than twice) when they first touch the DSCR.  Also, this
changes removes a bunch of ugly FTR_SECTION code.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_64.S

@@ -578,34 +578,15 @@ BEGIN_FTR_SECTION
 	ld	r7,DSCR_DEFAULT@toc(2)
 	ld	r0,THREAD_DSCR(r4)
 	cmpwi	r6,0
-	li	r8, FSCR_DSCR
 	bne	1f
 	ld	r0,0(r7)
-	b	3f
 1:
-  BEGIN_FTR_SECTION_NESTED(70)
-	mfspr	r6, SPRN_FSCR
-	or	r6, r6, r8
-	mtspr	SPRN_FSCR, r6
-    BEGIN_FTR_SECTION_NESTED(69)
-	mfspr	r6, SPRN_HFSCR
-	or	r6, r6, r8
-	mtspr	SPRN_HFSCR, r6
-    END_FTR_SECTION_NESTED(CPU_FTR_HVMODE, CPU_FTR_HVMODE, 69)
-	b	4f
-  END_FTR_SECTION_NESTED(CPU_FTR_ARCH_207S, CPU_FTR_ARCH_207S, 70)
-3:
-  BEGIN_FTR_SECTION_NESTED(70)
-	mfspr	r6, SPRN_FSCR
-	andc	r6, r6, r8
-	mtspr	SPRN_FSCR, r6
-    BEGIN_FTR_SECTION_NESTED(69)
-	mfspr	r6, SPRN_HFSCR
-	andc	r6, r6, r8
-	mtspr	SPRN_HFSCR, r6
-    END_FTR_SECTION_NESTED(CPU_FTR_HVMODE, CPU_FTR_HVMODE, 69)
-  END_FTR_SECTION_NESTED(CPU_FTR_ARCH_207S, CPU_FTR_ARCH_207S, 70)
-4:	cmpd	r0,r25
+BEGIN_FTR_SECTION_NESTED(70)
+	mfspr	r8, SPRN_FSCR
+	rldimi	r8, r6, FSCR_DSCR_LG, (63 - FSCR_DSCR_LG)
+	mtspr	SPRN_FSCR, r8
+END_FTR_SECTION_NESTED(CPU_FTR_ARCH_207S, CPU_FTR_ARCH_207S, 70)
+	cmpd	r0,r25
 	beq	2f
 	mtspr	SPRN_DSCR,r0
 2:

arch/powerpc/kernel/traps.c

@@ -1342,13 +1342,10 @@ void facility_unavailable_exception(struct pt_regs *regs)
 	if (status == FSCR_DSCR_LG) {
 		/* User is acessing the DSCR.  Set the inherit bit and allow
 		 * the user to set it directly in future by setting via the
-		 * H/FSCR DSCR bit.
+		 * FSCR DSCR bit.  We always leave HFSCR DSCR set.
 		 */
 		current->thread.dscr_inherit = 1;
-		if (hv)
-			mtspr(SPRN_HFSCR, value | HFSCR_DSCR);
-		else
-			mtspr(SPRN_FSCR,  value | FSCR_DSCR);
+		mtspr(SPRN_FSCR, value | FSCR_DSCR);
 		return;
 	}