linux/Documentation/powerpc/transactional_memory.rst
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   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