1============================
2Transactional Memory support
3============================
4
5POWER kernel support for this feature is currently limited to supporting
6its use by user programs.  It is not currently used by the kernel itself.
7
8This file aims to sum up how it is supported by Linux and what behaviour you
9can expect from your user programs.
10
11
12Basic overview
13==============
14
15Hardware Transactional Memory is supported on POWER8 processors, and is a
16feature that enables a different form of atomic memory access.  Several new
17instructions are presented to delimit transactions; transactions are
18guaranteed to either complete atomically or roll back and undo any partial
19changes.
20
21A simple transaction looks like this::
22
23  begin_move_money:
24    tbegin
25    beq   abort_handler
26
27    ld    r4, SAVINGS_ACCT(r3)
28    ld    r5, CURRENT_ACCT(r3)
29    subi  r5, r5, 1
30    addi  r4, r4, 1
31    std   r4, SAVINGS_ACCT(r3)
32    std   r5, CURRENT_ACCT(r3)
33
34    tend
35
36    b     continue
37
38  abort_handler:
39    ... test for odd failures ...
40
41    /* Retry the transaction if it failed because it conflicted with
42     * someone else: */
43    b     begin_move_money
44
45
46The 'tbegin' instruction denotes the start point, and 'tend' the end point.
47Between these points the processor is in 'Transactional' state; any memory
48references will complete in one go if there are no conflicts with other
49transactional or non-transactional accesses within the system.  In this
50example, the transaction completes as though it were normal straight-line code
51IF no other processor has touched SAVINGS_ACCT(r3) or CURRENT_ACCT(r3); an
52atomic move of money from the current account to the savings account has been
53performed.  Even though the normal ld/std instructions are used (note no
54lwarx/stwcx), either *both* SAVINGS_ACCT(r3) and CURRENT_ACCT(r3) will be
55updated, or neither will be updated.
56
57If, in the meantime, there is a conflict with the locations accessed by the
58transaction, the transaction will be aborted by the CPU.  Register and memory
59state will roll back to that at the 'tbegin', and control will continue from
60'tbegin+4'.  The branch to abort_handler will be taken this second time; the
61abort handler can check the cause of the failure, and retry.
62
63Checkpointed registers include all GPRs, FPRs, VRs/VSRs, LR, CCR/CR, CTR, FPCSR
64and a few other status/flag regs; see the ISA for details.
65
66Causes of transaction aborts
67============================
68
69- Conflicts with cache lines used by other processors
70- Signals
71- Context switches
72- See the ISA for full documentation of everything that will abort transactions.
73
74
75Syscalls
76========
77
78Syscalls made from within an active transaction will not be performed and the
79transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL
80| TM_CAUSE_PERSISTENT.
81
82Syscalls made from within a suspended transaction are performed as normal and
83the transaction is not explicitly doomed by the kernel.  However, what the
84kernel does to perform the syscall may result in the transaction being doomed
85by the hardware.  The syscall is performed in suspended mode so any side
86effects will be persistent, independent of transaction success or failure.  No
87guarantees are provided by the kernel about which syscalls will affect
88transaction success.
89
90Care must be taken when relying on syscalls to abort during active transactions
91if the calls are made via a library.  Libraries may cache values (which may
92give the appearance of success) or perform operations that cause transaction
93failure before entering the kernel (which may produce different failure codes).
94Examples are glibc's getpid() and lazy symbol resolution.
95
96
97Signals
98=======
99
100Delivery of signals (both sync and async) during transactions provides a second
101thread state (ucontext/mcontext) to represent the second transactional register
102state.  Signal delivery 'treclaim's to capture both register states, so signals
103abort transactions.  The usual ucontext_t passed to the signal handler
104represents the checkpointed/original register state; the signal appears to have
105arisen at 'tbegin+4'.
106
107If the sighandler ucontext has uc_link set, a second ucontext has been
108delivered.  For future compatibility the MSR.TS field should be checked to
109determine the transactional state -- if so, the second ucontext in uc->uc_link
110represents the active transactional registers at the point of the signal.
111
112For 64-bit processes, uc->uc_mcontext.regs->msr is a full 64-bit MSR and its TS
113field shows the transactional mode.
114
115For 32-bit processes, the mcontext's MSR register is only 32 bits; the top 32
116bits are stored in the MSR of the second ucontext, i.e. in
117uc->uc_link->uc_mcontext.regs->msr.  The top word contains the transactional
118state TS.
119
120However, basic signal handlers don't need to be aware of transactions
121and simply returning from the handler will deal with things correctly:
122
123Transaction-aware signal handlers can read the transactional register state
124from the second ucontext.  This will be necessary for crash handlers to
125determine, for example, the address of the instruction causing the SIGSEGV.
126
127Example signal handler::
128
129    void crash_handler(int sig, siginfo_t *si, void *uc)
130    {
131      ucontext_t *ucp = uc;
132      ucontext_t *transactional_ucp = ucp->uc_link;
133
134      if (ucp_link) {
135        u64 msr = ucp->uc_mcontext.regs->msr;
136        /* May have transactional ucontext! */
137  #ifndef __powerpc64__
138        msr |= ((u64)transactional_ucp->uc_mcontext.regs->msr) << 32;
139  #endif
140        if (MSR_TM_ACTIVE(msr)) {
141           /* Yes, we crashed during a transaction.  Oops. */
142   fprintf(stderr, "Transaction to be restarted at 0x%llx, but "
143                           "crashy instruction was at 0x%llx\n",
144                           ucp->uc_mcontext.regs->nip,
145                           transactional_ucp->uc_mcontext.regs->nip);
146        }
147      }
148
149      fix_the_problem(ucp->dar);
150    }
151
152When in an active transaction that takes a signal, we need to be careful with
153the stack.  It's possible that the stack has moved back up after the tbegin.
154The obvious case here is when the tbegin is called inside a function that
155returns before a tend.  In this case, the stack is part of the checkpointed
156transactional memory state.  If we write over this non transactionally or in
157suspend, we are in trouble because if we get a tm abort, the program counter and
158stack pointer will be back at the tbegin but our in memory stack won't be valid
159anymore.
160
161To avoid this, when taking a signal in an active transaction, we need to use
162the stack pointer from the checkpointed state, rather than the speculated
163state.  This ensures that the signal context (written tm suspended) will be
164written below the stack required for the rollback.  The transaction is aborted
165because of the treclaim, so any memory written between the tbegin and the
166signal will be rolled back anyway.
167
168For signals taken in non-TM or suspended mode, we use the
169normal/non-checkpointed stack pointer.
170
171Any transaction initiated inside a sighandler and suspended on return
172from the sighandler to the kernel will get reclaimed and discarded.
173
174Failure cause codes used by kernel
175==================================
176
177These are defined in <asm/reg.h>, and distinguish different reasons why the
178kernel aborted a transaction:
179
180 ====================== ================================
181 TM_CAUSE_RESCHED       Thread was rescheduled.
182 TM_CAUSE_TLBI          Software TLB invalid.
183 TM_CAUSE_FAC_UNAV      FP/VEC/VSX unavailable trap.
184 TM_CAUSE_SYSCALL       Syscall from active transaction.
185 TM_CAUSE_SIGNAL        Signal delivered.
186 TM_CAUSE_MISC          Currently unused.
187 TM_CAUSE_ALIGNMENT     Alignment fault.
188 TM_CAUSE_EMULATE       Emulation that touched memory.
189 ====================== ================================
190
191These can be checked by the user program's abort handler as TEXASR[0:7].  If
192bit 7 is set, it indicates that the error is considered persistent.  For example
193a TM_CAUSE_ALIGNMENT will be persistent while a TM_CAUSE_RESCHED will not.
194
195GDB
196===
197
198GDB and ptrace are not currently TM-aware.  If one stops during a transaction,
199it looks like the transaction has just started (the checkpointed state is
200presented).  The transaction cannot then be continued and will take the failure
201handler route.  Furthermore, the transactional 2nd register state will be
202inaccessible.  GDB can currently be used on programs using TM, but not sensibly
203in parts within transactions.
204
205POWER9
206======
207
208TM on POWER9 has issues with storing the complete register state. This
209is described in this commit::
210
211    commit 4bb3c7a0208fc13ca70598efd109901a7cd45ae7
212    Author: Paul Mackerras <paulus@ozlabs.org>
213    Date:   Wed Mar 21 21:32:01 2018 +1100
214    KVM: PPC: Book3S HV: Work around transactional memory bugs in POWER9
215
216To account for this different POWER9 chips have TM enabled in
217different ways.
218
219On POWER9N DD2.01 and below, TM is disabled. ie
220HWCAP2[PPC_FEATURE2_HTM] is not set.
221
222On POWER9N DD2.1 TM is configured by firmware to always abort a
223transaction when tm suspend occurs. So tsuspend will cause a
224transaction to be aborted and rolled back. Kernel exceptions will also
225cause the transaction to be aborted and rolled back and the exception
226will not occur. If userspace constructs a sigcontext that enables TM
227suspend, the sigcontext will be rejected by the kernel. This mode is
228advertised to users with HWCAP2[PPC_FEATURE2_HTM_NO_SUSPEND] set.
229HWCAP2[PPC_FEATURE2_HTM] is not set in this mode.
230
231On POWER9N DD2.2 and above, KVM and POWERVM emulate TM for guests (as
232described in commit 4bb3c7a0208f), hence TM is enabled for guests
233ie. HWCAP2[PPC_FEATURE2_HTM] is set for guest userspace. Guests that
234makes heavy use of TM suspend (tsuspend or kernel suspend) will result
235in traps into the hypervisor and hence will suffer a performance
236degradation. Host userspace has TM disabled
237ie. HWCAP2[PPC_FEATURE2_HTM] is not set. (although we make enable it
238at some point in the future if we bring the emulation into host
239userspace context switching).
240
241POWER9C DD1.2 and above are only available with POWERVM and hence
242Linux only runs as a guest. On these systems TM is emulated like on
243POWER9N DD2.2.
244
245Guest migration from POWER8 to POWER9 will work with POWER9N DD2.2 and
246POWER9C DD1.2. Since earlier POWER9 processors don't support TM
247emulation, migration from POWER8 to POWER9 is not supported there.
248
249Kernel implementation
250=====================
251
252h/rfid mtmsrd quirk
253-------------------
254
255As defined in the ISA, rfid has a quirk which is useful in early
256exception handling. When in a userspace transaction and we enter the
257kernel via some exception, MSR will end up as TM=0 and TS=01 (ie. TM
258off but TM suspended). Regularly the kernel will want change bits in
259the MSR and will perform an rfid to do this. In this case rfid can
260have SRR0 TM = 0 and TS = 00 (ie. TM off and non transaction) and the
261resulting MSR will retain TM = 0 and TS=01 from before (ie. stay in
262suspend). This is a quirk in the architecture as this would normally
263be a transition from TS=01 to TS=00 (ie. suspend -> non transactional)
264which is an illegal transition.
265
266This quirk is described the architecture in the definition of rfid
267with these lines:
268
269  if (MSR 29:31 ¬ = 0b010 | SRR1 29:31 ¬ = 0b000) then
270     MSR 29:31 <- SRR1 29:31
271
272hrfid and mtmsrd have the same quirk.
273
274The Linux kernel uses this quirk in its early exception handling.
275