From 6a404806dfceba9b154015705a9e647fa7749fd8 Mon Sep 17 00:00:00 2001 From: Michael Neuling Date: Wed, 27 Feb 2013 10:45:52 +0000 Subject: [PATCH] powerpc: Avoid link stack corruption in MMU on syscall entry path Currently we use the link register to branch up high in the early MMU on syscall entry path. Unfortunately, this trashes the link stack as the address we are going to is not associated with the earlier mflr. This patch simply converts us to used the count register (volatile over syscalls anyway) instead. This is much better at predicting in this scenario and doesn't trash link stack causing a bunch of additional branch mispredicts later. Benchmarking this on POWER8 saves a bunch of cycles on Anton's null syscall benchmark here: http://ozlabs.org/~anton/junkcode/null_syscall.c Signed-off-by: Michael Neuling Signed-off-by: Benjamin Herrenschmidt --- arch/powerpc/kernel/exceptions-64s.S | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S index a8a5361fb70c..87ef8f5ee5bc 100644 --- a/arch/powerpc/kernel/exceptions-64s.S +++ b/arch/powerpc/kernel/exceptions-64s.S @@ -74,13 +74,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE) \ mflr r10 ; \ ld r12,PACAKBASE(r13) ; \ LOAD_HANDLER(r12, system_call_entry_direct) ; \ - mtlr r12 ; \ + 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 */ \ - blr ; + bctr ; #else /* We can branch directly */ #define SYSCALL_PSERIES_2_DIRECT \