powerpc/tm: Abort syscalls in active transactions

This patch changes the syscall handler to doom (tabort) active
transactions when a syscall is made and return immediately without
performing the syscall.

Currently, the system call instruction automatically suspends an
active transaction which causes side effects to persist when an active
transaction fails.

This does change the kernel's behaviour, but in a way that was
documented as unsupported. It doesn't reduce functionality because
syscalls will still be performed after tsuspend. It also provides a
consistent interface and makes the behaviour of user code
substantially the same across powerpc and platforms that do not
support suspended transactions (e.g. x86 and s390).

Performance measurements using
http://ozlabs.org/~anton/junkcode/null_syscall.c
indicate the cost of a system call increases by about 0.5%.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Acked-By: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index afbc20019c2e..8ca9434c40e6 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -34,6 +34,7 @@
 #include <asm/ftrace.h>
 #include <asm/hw_irq.h>
 #include <asm/context_tracking.h>
+#include <asm/tm.h>
 
 /*
  * System calls.
@@ -145,6 +146,24 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
         andi.   r11,r10,_TIF_SYSCALL_DOTRACE
         bne     syscall_dotrace
 .Lsyscall_dotrace_cont:
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+BEGIN_FTR_SECTION
+        b       1f
+END_FTR_SECTION_IFCLR(CPU_FTR_TM)
+        extrdi. r11, r12, 1, (63-MSR_TS_T_LG) /* transaction active? */
+        beq+    1f
+
+        /* Doom the transaction and don't perform the syscall: */
+        mfmsr   r11
+        li      r12, 1
+        rldimi  r11, r12, MSR_TM_LG, 63-MSR_TM_LG
+        mtmsrd  r11, 0
+        li      r11, (TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT)
+        TABORT(R11)
+
+        b       .Lsyscall_exit
+1:
+#endif
         cmpldi  0,r0,NR_syscalls
         bge-    syscall_enosys