powerpc/64: Avoid restore_math call if possible in syscall exit
The syscall exit code that branches to restore_math is quite heavy on
Book3S, consisting of 2 mtmsr instructions. Threads that don't use both
FP and vector can get caught here if the kernel ever uses FP or vector.
Lazy-FP/vec context switching also trips this case.
So check for lazy FP and vector before switching RI for restore_math.
Move most of this case out of line.
For threads that do want to restore math registers, the MSR switches are
still suboptimal. Future direction may be to use a soft-RI bit to avoid
MSR switches in kernel (similar to soft-EE), but for now at least the
no-restore
POWER9 context switch rate increases by about 5% due to sched_yield(2)
return performance. I haven't constructed a test to measure the syscall
cost.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index bfbad08..6f70ea8 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -210,27 +210,17 @@
andi. r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
bne- syscall_exit_work
- andi. r0,r8,MSR_FP
- beq 2f
+ /* If MSR_FP and MSR_VEC are set in user msr, then no need to restore */
+ li r7,MSR_FP
#ifdef CONFIG_ALTIVEC
- andis. r0,r8,MSR_VEC@h
- bne 3f
+ oris r7,r7,MSR_VEC@h
#endif
-2: addi r3,r1,STACK_FRAME_OVERHEAD
-#ifdef CONFIG_PPC_BOOK3S
- li r10,MSR_RI
- mtmsrd r10,1 /* Restore RI */
-#endif
- bl restore_math
-#ifdef CONFIG_PPC_BOOK3S
- li r11,0
- mtmsrd r11,1
-#endif
- ld r8,_MSR(r1)
- ld r3,RESULT(r1)
- li r11,-MAX_ERRNO
+ and r0,r8,r7
+ cmpd r0,r7
+ bne syscall_restore_math
+.Lsyscall_restore_math_cont:
-3: cmpld r3,r11
+ cmpld r3,r11
ld r5,_CCR(r1)
bge- syscall_error
.Lsyscall_error_cont:
@@ -263,7 +253,41 @@
neg r3,r3
std r5,_CCR(r1)
b .Lsyscall_error_cont
-
+
+syscall_restore_math:
+ /*
+ * Some initial tests from restore_math to avoid the heavyweight
+ * C code entry and MSR manipulations.
+ */
+ LOAD_REG_IMMEDIATE(r0, MSR_TS_MASK)
+ and. r0,r0,r8
+ bne 1f
+
+ ld r7,PACACURRENT(r13)
+ lbz r0,THREAD+THREAD_LOAD_FP(r7)
+#ifdef CONFIG_ALTIVEC
+ lbz r6,THREAD+THREAD_LOAD_VEC(r7)
+ add r0,r0,r6
+#endif
+ cmpdi r0,0
+ beq .Lsyscall_restore_math_cont
+
+1: addi r3,r1,STACK_FRAME_OVERHEAD
+#ifdef CONFIG_PPC_BOOK3S
+ li r10,MSR_RI
+ mtmsrd r10,1 /* Restore RI */
+#endif
+ bl restore_math
+#ifdef CONFIG_PPC_BOOK3S
+ li r11,0
+ mtmsrd r11,1
+#endif
+ /* Restore volatiles, reload MSR from updated one */
+ ld r8,_MSR(r1)
+ ld r3,RESULT(r1)
+ li r11,-MAX_ERRNO
+ b .Lsyscall_restore_math_cont
+
/* Traced system call support */
syscall_dotrace:
bl save_nvgprs
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index baae104..5cbb8b1 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -511,6 +511,10 @@
{
unsigned long msr;
+ /*
+ * Syscall exit makes a similar initial check before branching
+ * to restore_math. Keep them in synch.
+ */
if (!msr_tm_active(regs->msr) &&
!current->thread.load_fp && !loadvec(current->thread))
return;