From e7fda7e569e1776d4dccbcef52d34882b62b0654 Mon Sep 17 00:00:00 2001 From: Michael Ellerman Date: Thu, 29 Nov 2018 17:42:24 +1100 Subject: [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit() Currently in system_call_exit() we have an optimisation where we disable MSR_RI (recoverable interrupt) and MSR_EE (external interrupt enable) in a single mtmsrd instruction. Unfortunately this will no longer work with THREAD_INFO_IN_TASK, because then the load of TI_FLAGS might fault and faulting with MSR_RI clear is treated as an unrecoverable exception which leads to a panic(). So change the code to only clear MSR_EE prior to loading TI_FLAGS, leaving the clear of MSR_RI until later. We have some latitude in where do the clear of MSR_RI. A bit of experimentation has shown that this location gives the least slow down. This still causes a noticeable slow down in our null_syscall performance. On a Power9 DD2.2: Before After Delta Delta % 955 cycles 999 cycles -44 -4.6% On the plus side this does simplify the code somewhat, because we don't have to reenable MSR_RI on the restore_math() or syscall_exit_work() paths which was necessitated previously by the optimisation. Reviewed-by: Nicholas Piggin Signed-off-by: Michael Ellerman --- arch/powerpc/kernel/entry_64.S | 34 ++++++++++++++-------------------- 1 file changed, 14 insertions(+), 20 deletions(-) diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index a2c168b395d2..c17c1bed6148 100644 --- a/arch/powerpc/kernel/entry_64.S +++ b/arch/powerpc/kernel/entry_64.S @@ -236,18 +236,14 @@ system_call_exit: /* * Disable interrupts so current_thread_info()->flags can't change, * and so that we don't get interrupted after loading SRR0/1. + * + * Leave MSR_RI enabled for now, because with THREAD_INFO_IN_TASK we + * could fault on the load of the TI_FLAGS below. */ #ifdef CONFIG_PPC_BOOK3E wrteei 0 #else - /* - * For performance reasons we clear RI the same time that we - * clear EE. We only need to clear RI just before we restore r13 - * below, but batching it with EE saves us one expensive mtmsrd call. - * We have to be careful to restore RI if we branch anywhere from - * here (eg syscall_exit_work). - */ - li r11,0 + li r11,MSR_RI mtmsrd r11,1 #endif /* CONFIG_PPC_BOOK3E */ @@ -263,15 +259,7 @@ system_call_exit: bne 3f #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 @@ -287,6 +275,16 @@ END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS) andi. r6,r8,MSR_PR ld r4,_LINK(r1) +#ifdef CONFIG_PPC_BOOK3S + /* + * Clear MSR_RI, MSR_EE is already and remains disabled. We could do + * this later, but testing shows that doing it here causes less slow + * down than doing it closer to the rfid. + */ + li r11,0 + mtmsrd r11,1 +#endif + beq- 1f ACCOUNT_CPU_USER_EXIT(r13, r11, r12) @@ -363,10 +361,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR) b .Lsyscall_exit .Lsyscall_exit_work: -#ifdef CONFIG_PPC_BOOK3S - li r10,MSR_RI - mtmsrd r10,1 /* Restore RI */ -#endif /* If TIF_RESTOREALL is set, don't scribble on either r3 or ccr. If TIF_NOERROR is set, just save r3 as it is. */ -- 2.11.0