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powerpc/tm: Abort syscalls in active transactions

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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>
This commit is contained in:
Sam bobroff
2015-06-12 11:06:32 +10:00
committed by Michael Ellerman
parent b5926430df
commit b4b56f9eca
8 changed files with 66 additions and 25 deletions
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4
arch/powerpc/kernel/cputable.c
View File

@@ -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 |\

35
arch/powerpc/kernel/entry_64.S
View File

@@ -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)