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 very early without
performing the syscall and keeping side effects to a minimum (no CPU
accounting or system call tracing is performed). Also included is a
new HWCAP2 bit, PPC_FEATURE2_HTM_NOSC, to indicate this
behaviour to userspace.

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 as
syscalls will still be performed after tsuspend; it just requires that
the transaction be explicitly suspended.  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 normal (non-aborted) system call increases by about 0.25%.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

5 files changed, 46 insertions, 6 deletions

diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
index 6367b83..4994648 100644
--- a/arch/powerpc/include/asm/cputable.h
+++ b/arch/powerpc/include/asm/cputable.h
@@ -242,11 +242,13 @@ enum {
 
 /* We only set the TM feature if the kernel was compiled with TM supprt */
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
-#define CPU_FTR_TM_COMP		CPU_FTR_TM
-#define PPC_FEATURE2_HTM_COMP	PPC_FEATURE2_HTM
+#define CPU_FTR_TM_COMP			CPU_FTR_TM
+#define PPC_FEATURE2_HTM_COMP		PPC_FEATURE2_HTM
+#define PPC_FEATURE2_HTM_NOSC_COMP	PPC_FEATURE2_HTM_NOSC
 #else
-#define CPU_FTR_TM_COMP		0
-#define PPC_FEATURE2_HTM_COMP	0
+#define CPU_FTR_TM_COMP			0
+#define PPC_FEATURE2_HTM_COMP		0
+#define PPC_FEATURE2_HTM_NOSC_COMP	0
 #endif
 
 /* We need to mark all pages as being coherent if we're SMP or we have a
diff --git a/arch/powerpc/include/uapi/asm/cputable.h b/arch/powerpc/include/uapi/asm/cputable.h
index de2c0e4..4368604 100644
--- a/arch/powerpc/include/uapi/asm/cputable.h
+++ b/arch/powerpc/include/uapi/asm/cputable.h
@@ -42,5 +42,6 @@
 #define PPC_FEATURE2_ISEL		0x08000000
 #define PPC_FEATURE2_TAR		0x04000000
 #define PPC_FEATURE2_VEC_CRYPTO		0x02000000
+#define PPC_FEATURE2_HTM_NOSC		0x01000000
 
 #endif /* _UAPI__ASM_POWERPC_CPUTABLE_H */
diff --git a/arch/powerpc/include/uapi/asm/tm.h b/arch/powerpc/include/uapi/asm/tm.h
index 5d836b7..5047659 100644
--- a/arch/powerpc/include/uapi/asm/tm.h
+++ b/arch/powerpc/include/uapi/asm/tm.h
@@ -11,7 +11,7 @@
 #define TM_CAUSE_RESCHED	0xde
 #define TM_CAUSE_TLBI		0xdc
 #define TM_CAUSE_FAC_UNAV	0xda
-#define TM_CAUSE_SYSCALL	0xd8  /* future use */
+#define TM_CAUSE_SYSCALL	0xd8
 #define TM_CAUSE_MISC		0xd6  /* future use */
 #define TM_CAUSE_SIGNAL		0xd4
 #define TM_CAUSE_ALIGNMENT	0xd2
diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
index 60262fd..7d80bfd 100644
--- a/arch/powerpc/kernel/cputable.c
+++ b/arch/powerpc/kernel/cputable.c
@@ -108,7 +108,9 @@ extern void __restore_cpu_e6500(void);
 				 PPC_FEATURE_TRUE_LE | \
 				 PPC_FEATURE_PSERIES_PERFMON_COMPAT)
 #define COMMON_USER2_POWER8	(PPC_FEATURE2_ARCH_2_07 | \
-				 PPC_FEATURE2_HTM_COMP | PPC_FEATURE2_DSCR | \
+				 PPC_FEATURE2_HTM_COMP | \
+				 PPC_FEATURE2_HTM_NOSC_COMP | \
+				 PPC_FEATURE2_DSCR | \
 				 PPC_FEATURE2_ISEL | PPC_FEATURE2_TAR | \
 				 PPC_FEATURE2_VEC_CRYPTO)
 #define COMMON_USER_PA6T	(COMMON_USER_PPC64 | PPC_FEATURE_PA6T |\
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 278888e..579e0f9 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.
@@ -51,6 +52,12 @@ exception_marker:
 
 	.globl system_call_common
 system_call_common:
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+BEGIN_FTR_SECTION
+	extrdi.	r10, r12, 1, (63-MSR_TS_T_LG) /* transaction active? */
+	bne	tabort_syscall
+END_FTR_SECTION_IFSET(CPU_FTR_TM)
+#endif
 	andi.	r10,r12,MSR_PR
 	mr	r10,r1
 	addi	r1,r1,-INT_FRAME_SIZE
@@ -311,6 +318,34 @@ syscall_exit_work:
 	bl	do_syscall_trace_leave
 	b	ret_from_except
 
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+tabort_syscall:
+	/* Firstly we need to enable TM in the kernel */
+	mfmsr	r10
+	li	r13, 1
+	rldimi	r10, r13, MSR_TM_LG, 63-MSR_TM_LG
+	mtmsrd	r10, 0
+
+	/* tabort, this dooms the transaction, nothing else */
+	li	r13, (TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT)
+	TABORT(R13)
+
+	/*
+	 * Return directly to userspace. We have corrupted user register state,
+	 * but userspace will never see that register state. Execution will
+	 * resume after the tbegin of the aborted transaction with the
+	 * checkpointed register state.
+	 */
+	li	r13, MSR_RI
+	andc	r10, r10, r13
+	mtmsrd	r10, 1
+	mtspr	SPRN_SRR0, r11
+	mtspr	SPRN_SRR1, r12
+
+	rfid
+	b	.	/* prevent speculative execution */
+#endif
+
 /* Save non-volatile GPRs, if not already saved. */
 _GLOBAL(save_nvgprs)
 	ld	r11,_TRAP(r1)