powerpc/64: Optimise syscall entry for virtual, relocatable case

The mflr r10 instruction was left over from when the code used LR to
branch to system_call_entry from the exception handler. That was
changed by commit 6a404806dfce ("powerpc: Avoid link stack corruption in
MMU on syscall entry path") to use the count register. The value is
never used now, so mflr can be removed, and r10 can be used for storage
rather than spilling to the SPR scratch register.

The scratch register spill causes a long pipeline stall due to the SPR
read after write. This change brings getppid syscall cost from 406 to
376 cycles on POWER8. getppid for non-relocatable case is 371 cycles.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 21ab5dd..3d1f3cf 100644 (file)
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -62,14 +62,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)                              \
         * is volatile across system calls.
         */
 #define SYSCALL_PSERIES_2_DIRECT                               \
-       mflr    r10 ;                                           \
        LOAD_HANDLER(r12, system_call_entry) ;                  \
        mtctr   r12 ;                                           \
        mfspr   r12,SPRN_SRR1 ;                                 \
-       /* Re-use of r13... No spare regs to do this */ \
-       li      r13,MSR_RI ;                                    \
-       mtmsrd  r13,1 ;                                         \
-       GET_PACA(r13) ; /* get r13 back */                      \
+       li      r10,MSR_RI ;                                    \
+       mtmsrd  r10,1 ;                                         \
        bctr ;
 #else
        /* We can branch directly */