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Michael Neulingdb8ff902013-02-13 16:21:45 +00001Transactional Memory support
2============================
3
4POWER kernel support for this feature is currently limited to supporting
5its use by user programs. It is not currently used by the kernel itself.
6
7This file aims to sum up how it is supported by Linux and what behaviour you
8can expect from your user programs.
9
10
11Basic overview
12==============
13
14Hardware Transactional Memory is supported on POWER8 processors, and is a
15feature that enables a different form of atomic memory access. Several new
16instructions are presented to delimit transactions; transactions are
17guaranteed to either complete atomically or roll back and undo any partial
18changes.
19
20A simple transaction looks like this:
21
22begin_move_money:
23 tbegin
24 beq abort_handler
25
26 ld r4, SAVINGS_ACCT(r3)
27 ld r5, CURRENT_ACCT(r3)
28 subi r5, r5, 1
29 addi r4, r4, 1
30 std r4, SAVINGS_ACCT(r3)
31 std r5, CURRENT_ACCT(r3)
32
33 tend
34
35 b continue
36
37abort_handler:
38 ... test for odd failures ...
39
40 /* Retry the transaction if it failed because it conflicted with
41 * someone else: */
42 b begin_move_money
43
44
45The 'tbegin' instruction denotes the start point, and 'tend' the end point.
46Between these points the processor is in 'Transactional' state; any memory
47references will complete in one go if there are no conflicts with other
48transactional or non-transactional accesses within the system. In this
49example, the transaction completes as though it were normal straight-line code
50IF no other processor has touched SAVINGS_ACCT(r3) or CURRENT_ACCT(r3); an
51atomic move of money from the current account to the savings account has been
52performed. Even though the normal ld/std instructions are used (note no
53lwarx/stwcx), either *both* SAVINGS_ACCT(r3) and CURRENT_ACCT(r3) will be
54updated, or neither will be updated.
55
56If, in the meantime, there is a conflict with the locations accessed by the
57transaction, the transaction will be aborted by the CPU. Register and memory
58state will roll back to that at the 'tbegin', and control will continue from
59'tbegin+4'. The branch to abort_handler will be taken this second time; the
60abort handler can check the cause of the failure, and retry.
61
62Checkpointed registers include all GPRs, FPRs, VRs/VSRs, LR, CCR/CR, CTR, FPCSR
63and a few other status/flag regs; see the ISA for details.
64
65Causes of transaction aborts
66============================
67
68- Conflicts with cache lines used by other processors
69- Signals
70- Context switches
71- See the ISA for full documentation of everything that will abort transactions.
72
73
74Syscalls
75========
76
77Performing syscalls from within transaction is not recommended, and can lead
78to unpredictable results.
79
80Syscalls do not by design abort transactions, but beware: The kernel code will
81not be running in transactional state. The effect of syscalls will always
82remain visible, but depending on the call they may abort your transaction as a
83side-effect, read soon-to-be-aborted transactional data that should not remain
84invisible, etc. If you constantly retry a transaction that constantly aborts
85itself by calling a syscall, you'll have a livelock & make no progress.
86
87Simple syscalls (e.g. sigprocmask()) "could" be OK. Even things like write()
88from, say, printf() should be OK as long as the kernel does not access any
89memory that was accessed transactionally.
90
91Consider any syscalls that happen to work as debug-only -- not recommended for
92production use. Best to queue them up till after the transaction is over.
93
94
95Signals
96=======
97
98Delivery of signals (both sync and async) during transactions provides a second
99thread state (ucontext/mcontext) to represent the second transactional register
100state. Signal delivery 'treclaim's to capture both register states, so signals
101abort transactions. The usual ucontext_t passed to the signal handler
102represents the checkpointed/original register state; the signal appears to have
103arisen at 'tbegin+4'.
104
105If the sighandler ucontext has uc_link set, a second ucontext has been
106delivered. For future compatibility the MSR.TS field should be checked to
107determine the transactional state -- if so, the second ucontext in uc->uc_link
108represents the active transactional registers at the point of the signal.
109
110For 64-bit processes, uc->uc_mcontext.regs->msr is a full 64-bit MSR and its TS
111field shows the transactional mode.
112
113For 32-bit processes, the mcontext's MSR register is only 32 bits; the top 32
114bits are stored in the MSR of the second ucontext, i.e. in
115uc->uc_link->uc_mcontext.regs->msr. The top word contains the transactional
116state TS.
117
118However, basic signal handlers don't need to be aware of transactions
119and simply returning from the handler will deal with things correctly:
120
121Transaction-aware signal handlers can read the transactional register state
122from the second ucontext. This will be necessary for crash handlers to
123determine, for example, the address of the instruction causing the SIGSEGV.
124
125Example signal handler:
126
127 void crash_handler(int sig, siginfo_t *si, void *uc)
128 {
129 ucontext_t *ucp = uc;
130 ucontext_t *transactional_ucp = ucp->uc_link;
131
132 if (ucp_link) {
133 u64 msr = ucp->uc_mcontext.regs->msr;
134 /* May have transactional ucontext! */
135#ifndef __powerpc64__
136 msr |= ((u64)transactional_ucp->uc_mcontext.regs->msr) << 32;
137#endif
138 if (MSR_TM_ACTIVE(msr)) {
139 /* Yes, we crashed during a transaction. Oops. */
140 fprintf(stderr, "Transaction to be restarted at 0x%llx, but "
141 "crashy instruction was at 0x%llx\n",
142 ucp->uc_mcontext.regs->nip,
143 transactional_ucp->uc_mcontext.regs->nip);
144 }
145 }
146
147 fix_the_problem(ucp->dar);
148 }
149
Michael Neuling2b3f8e82013-05-26 18:09:41 +0000150When in an active transaction that takes a signal, we need to be careful with
151the stack. It's possible that the stack has moved back up after the tbegin.
152The obvious case here is when the tbegin is called inside a function that
153returns before a tend. In this case, the stack is part of the checkpointed
154transactional memory state. If we write over this non transactionally or in
155suspend, we are in trouble because if we get a tm abort, the program counter and
156stack pointer will be back at the tbegin but our in memory stack won't be valid
157anymore.
158
159To avoid this, when taking a signal in an active transaction, we need to use
160the stack pointer from the checkpointed state, rather than the speculated
161state. This ensures that the signal context (written tm suspended) will be
162written below the stack required for the rollback. The transaction is aborted
163becuase of the treclaim, so any memory written between the tbegin and the
164signal will be rolled back anyway.
165
166For signals taken in non-TM or suspended mode, we use the
167normal/non-checkpointed stack pointer.
168
Michael Neulingdb8ff902013-02-13 16:21:45 +0000169
170Failure cause codes used by kernel
171==================================
172
173These are defined in <asm/reg.h>, and distinguish different reasons why the
174kernel aborted a transaction:
175
176 TM_CAUSE_RESCHED Thread was rescheduled.
Michael Neuling24b92372013-05-26 18:09:38 +0000177 TM_CAUSE_TLBI Software TLB invalide.
Michael Neulingdb8ff902013-02-13 16:21:45 +0000178 TM_CAUSE_FAC_UNAV FP/VEC/VSX unavailable trap.
179 TM_CAUSE_SYSCALL Currently unused; future syscalls that must abort
180 transactions for consistency will use this.
181 TM_CAUSE_SIGNAL Signal delivered.
182 TM_CAUSE_MISC Currently unused.
Michael Neuling6ce6c622013-05-26 18:09:39 +0000183 TM_CAUSE_ALIGNMENT Alignment fault.
184 TM_CAUSE_EMULATE Emulation that touched memory.
Michael Neulingdb8ff902013-02-13 16:21:45 +0000185
Michael Neuling6ce6c622013-05-26 18:09:39 +0000186These can be checked by the user program's abort handler as TEXASR[0:7]. If
187bit 7 is set, it indicates that the error is consider persistent. For example
188a TM_CAUSE_ALIGNMENT will be persistent while a TM_CAUSE_RESCHED will not.q
Michael Neulingdb8ff902013-02-13 16:21:45 +0000189
190GDB
191===
192
193GDB and ptrace are not currently TM-aware. If one stops during a transaction,
194it looks like the transaction has just started (the checkpointed state is
195presented). The transaction cannot then be continued and will take the failure
196handler route. Furthermore, the transactional 2nd register state will be
197inaccessible. GDB can currently be used on programs using TM, but not sensibly
198in parts within transactions.