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 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -210,27 +210,17 @@ system_call:                      /* label this so stack traces look sane */
        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 @@ syscall_error:
        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 (file)
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -511,6 +511,10 @@ void restore_math(struct pt_regs *regs)
 {
        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;