powerpc: Create context switch helpers save_sprs() and restore_sprs()

Move all our context switch SPR save and restore code into two
helpers. We do a few optimisations:

- Group all mfsprs and all mtsprs. In many cases an mtspr sets a
scoreboarding bit that an mfspr waits on, so the current practise of
mfspr A; mtspr A; mfpsr B; mtspr B is the worst scheduling we can
do.

- SPR writes are slow, so check that the value is changing before
writing it.

A context switch microbenchmark using yield():

http://ozlabs.org/~anton/junkcode/context_switch2.c

./context_switch2 --test=yield 0 0

shows an improvement of almost 10% on POWER8.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/processor.h

@@ -294,6 +294,7 @@ struct thread_struct {
 #endif
 #ifdef CONFIG_PPC64
 	unsigned long	dscr;
+	unsigned long	fscr;
 	/*
 	 * This member element dscr_inherit indicates that the process
 	 * has explicitly attempted and changed the DSCR register value

arch/powerpc/include/asm/switch_to.h

@@ -15,17 +15,6 @@ extern struct task_struct *__switch_to(struct task_struct *,
 struct thread_struct;
 extern struct task_struct *_switch(struct thread_struct *prev,
 				   struct thread_struct *next);
-#ifdef CONFIG_PPC_BOOK3S_64
-static inline void save_early_sprs(struct thread_struct *prev)
-{
-	if (cpu_has_feature(CPU_FTR_ARCH_207S))
-		prev->tar = mfspr(SPRN_TAR);
-	if (cpu_has_feature(CPU_FTR_DSCR))
-		prev->dscr = mfspr(SPRN_DSCR);
-}
-#else
-static inline void save_early_sprs(struct thread_struct *prev) {}
-#endif
 
 extern void enable_kernel_fp(void);
 extern void enable_kernel_altivec(void);

arch/powerpc/kernel/entry_64.S

@@ -453,29 +453,12 @@ _GLOBAL(_switch)
 	SAVE_8GPRS(14, r1)
 	SAVE_10GPRS(22, r1)
 	mflr	r20		/* Return to switch caller */
-#ifdef CONFIG_ALTIVEC
-BEGIN_FTR_SECTION
-	mfspr	r24,SPRN_VRSAVE	/* save vrsave register value */
-	std	r24,THREAD_VRSAVE(r3)
-END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
-#endif /* CONFIG_ALTIVEC */
+
 	std	r20,_NIP(r1)
 	mfcr	r23
 	std	r23,_CCR(r1)
 	std	r1,KSP(r3)	/* Set old stack pointer */
 
-#ifdef CONFIG_PPC_BOOK3S_64
-BEGIN_FTR_SECTION
-	/* Event based branch registers */
-	mfspr	r0, SPRN_BESCR
-	std	r0, THREAD_BESCR(r3)
-	mfspr	r0, SPRN_EBBHR
-	std	r0, THREAD_EBBHR(r3)
-	mfspr	r0, SPRN_EBBRR
-	std	r0, THREAD_EBBRR(r3)
-END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
-#endif
-
 #ifdef CONFIG_SMP
 	/* We need a sync somewhere here to make sure that if the
 	 * previous task gets rescheduled on another CPU, it sees all
@@ -563,47 +546,6 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
 	mr	r1,r8		/* start using new stack pointer */
 	std	r7,PACAKSAVE(r13)
 
-#ifdef CONFIG_PPC_BOOK3S_64
-BEGIN_FTR_SECTION
-	/* Event based branch registers */
-	ld	r0, THREAD_BESCR(r4)
-	mtspr	SPRN_BESCR, r0
-	ld	r0, THREAD_EBBHR(r4)
-	mtspr	SPRN_EBBHR, r0
-	ld	r0, THREAD_EBBRR(r4)
-	mtspr	SPRN_EBBRR, r0
-
-	ld	r0,THREAD_TAR(r4)
-	mtspr	SPRN_TAR,r0
-END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
-#endif
-
-#ifdef CONFIG_ALTIVEC
-BEGIN_FTR_SECTION
-	ld	r0,THREAD_VRSAVE(r4)
-	mtspr	SPRN_VRSAVE,r0		/* if G4, restore VRSAVE reg */
-END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
-#endif /* CONFIG_ALTIVEC */
-#ifdef CONFIG_PPC64
-BEGIN_FTR_SECTION
-	lwz	r6,THREAD_DSCR_INHERIT(r4)
-	ld	r0,THREAD_DSCR(r4)
-	cmpwi	r6,0
-	bne	1f
-	ld	r0,PACA_DSCR_DEFAULT(r13)
-1:
-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:
-END_FTR_SECTION_IFSET(CPU_FTR_DSCR)
-#endif
-
 	ld	r6,_CCR(r1)
 	mtcrf	0xFF,r6
 

arch/powerpc/kernel/process.c

@@ -742,6 +742,73 @@ void restore_tm_state(struct pt_regs *regs)
 #define __switch_to_tm(prev)
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
 
+static inline void save_sprs(struct thread_struct *t)
+{
+#ifdef CONFIG_ALTIVEC
+	if (cpu_has_feature(cpu_has_feature(CPU_FTR_ALTIVEC)))
+		t->vrsave = mfspr(SPRN_VRSAVE);
+#endif
+#ifdef CONFIG_PPC_BOOK3S_64
+	if (cpu_has_feature(CPU_FTR_DSCR))
+		t->dscr = mfspr(SPRN_DSCR);
+
+	if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
+		t->bescr = mfspr(SPRN_BESCR);
+		t->ebbhr = mfspr(SPRN_EBBHR);
+		t->ebbrr = mfspr(SPRN_EBBRR);
+
+		t->fscr = mfspr(SPRN_FSCR);
+
+		/*
+		 * Note that the TAR is not available for use in the kernel.
+		 * (To provide this, the TAR should be backed up/restored on
+		 * exception entry/exit instead, and be in pt_regs.  FIXME,
+		 * this should be in pt_regs anyway (for debug).)
+		 */
+		t->tar = mfspr(SPRN_TAR);
+	}
+#endif
+}
+
+static inline void restore_sprs(struct thread_struct *old_thread,
+				struct thread_struct *new_thread)
+{
+#ifdef CONFIG_ALTIVEC
+	if (cpu_has_feature(CPU_FTR_ALTIVEC) &&
+	    old_thread->vrsave != new_thread->vrsave)
+		mtspr(SPRN_VRSAVE, new_thread->vrsave);
+#endif
+#ifdef CONFIG_PPC_BOOK3S_64
+	if (cpu_has_feature(CPU_FTR_DSCR)) {
+		u64 dscr = get_paca()->dscr_default;
+		u64 fscr = old_thread->fscr & ~FSCR_DSCR;
+
+		if (new_thread->dscr_inherit) {
+			dscr = new_thread->dscr;
+			fscr |= FSCR_DSCR;
+		}
+
+		if (old_thread->dscr != dscr)
+			mtspr(SPRN_DSCR, dscr);
+
+		if (old_thread->fscr != fscr)
+			mtspr(SPRN_FSCR, fscr);
+	}
+
+	if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
+		if (old_thread->bescr != new_thread->bescr)
+			mtspr(SPRN_BESCR, new_thread->bescr);
+		if (old_thread->ebbhr != new_thread->ebbhr)
+			mtspr(SPRN_EBBHR, new_thread->ebbhr);
+		if (old_thread->ebbrr != new_thread->ebbrr)
+			mtspr(SPRN_EBBRR, new_thread->ebbrr);
+
+		if (old_thread->tar != new_thread->tar)
+			mtspr(SPRN_TAR, new_thread->tar);
+	}
+#endif
+}
+
 struct task_struct *__switch_to(struct task_struct *prev,
 	struct task_struct *new)
 {
@@ -751,17 +818,16 @@ struct task_struct *__switch_to(struct task_struct *prev,
 	struct ppc64_tlb_batch *batch;
 #endif
 
+	new_thread = &new->thread;
+	old_thread = &current->thread;
+
 	WARN_ON(!irqs_disabled());
 
-	/* Back up the TAR and DSCR across context switches.
-	 * Note that the TAR is not available for use in the kernel.  (To
-	 * provide this, the TAR should be backed up/restored on exception
-	 * entry/exit instead, and be in pt_regs.  FIXME, this should be in
-	 * pt_regs anyway (for debug).)
-	 * Save the TAR and DSCR here before we do treclaim/trecheckpoint as
-	 * these will change them.
+	/*
+	 * We need to save SPRs before treclaim/trecheckpoint as these will
+	 * change a number of them.
 	 */
-	save_early_sprs(&prev->thread);
+	save_sprs(&prev->thread);
 
 	__switch_to_tm(prev);
 
@@ -844,10 +910,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
 #endif /* CONFIG_HAVE_HW_BREAKPOINT */
 #endif
 
-
-	new_thread = &new->thread;
-	old_thread = &current->thread;
-
 #ifdef CONFIG_PPC64
 	/*
 	 * Collect processor utilization data per process
@@ -883,6 +945,10 @@ struct task_struct *__switch_to(struct task_struct *prev,
 
 	last = _switch(old_thread, new_thread);
 
+	/* Need to recalculate these after calling _switch() */
+	old_thread = &last->thread;
+	new_thread = &current->thread;
+
 #ifdef CONFIG_PPC_BOOK3S_64
 	if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
 		current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
@@ -891,6 +957,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
 	}
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
+	restore_sprs(old_thread, new_thread);
+
 	return last;
 }