Update Linux to v5.4.2
Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
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+============================
+Transactional Memory support
+============================
+
+POWER kernel support for this feature is currently limited to supporting
+its use by user programs. It is not currently used by the kernel itself.
+
+This file aims to sum up how it is supported by Linux and what behaviour you
+can expect from your user programs.
+
+
+Basic overview
+==============
+
+Hardware Transactional Memory is supported on POWER8 processors, and is a
+feature that enables a different form of atomic memory access. Several new
+instructions are presented to delimit transactions; transactions are
+guaranteed to either complete atomically or roll back and undo any partial
+changes.
+
+A simple transaction looks like this::
+
+ begin_move_money:
+ tbegin
+ beq abort_handler
+
+ ld r4, SAVINGS_ACCT(r3)
+ ld r5, CURRENT_ACCT(r3)
+ subi r5, r5, 1
+ addi r4, r4, 1
+ std r4, SAVINGS_ACCT(r3)
+ std r5, CURRENT_ACCT(r3)
+
+ tend
+
+ b continue
+
+ abort_handler:
+ ... test for odd failures ...
+
+ /* Retry the transaction if it failed because it conflicted with
+ * someone else: */
+ b begin_move_money
+
+
+The 'tbegin' instruction denotes the start point, and 'tend' the end point.
+Between these points the processor is in 'Transactional' state; any memory
+references will complete in one go if there are no conflicts with other
+transactional or non-transactional accesses within the system. In this
+example, the transaction completes as though it were normal straight-line code
+IF no other processor has touched SAVINGS_ACCT(r3) or CURRENT_ACCT(r3); an
+atomic move of money from the current account to the savings account has been
+performed. Even though the normal ld/std instructions are used (note no
+lwarx/stwcx), either *both* SAVINGS_ACCT(r3) and CURRENT_ACCT(r3) will be
+updated, or neither will be updated.
+
+If, in the meantime, there is a conflict with the locations accessed by the
+transaction, the transaction will be aborted by the CPU. Register and memory
+state will roll back to that at the 'tbegin', and control will continue from
+'tbegin+4'. The branch to abort_handler will be taken this second time; the
+abort handler can check the cause of the failure, and retry.
+
+Checkpointed registers include all GPRs, FPRs, VRs/VSRs, LR, CCR/CR, CTR, FPCSR
+and a few other status/flag regs; see the ISA for details.
+
+Causes of transaction aborts
+============================
+
+- Conflicts with cache lines used by other processors
+- Signals
+- Context switches
+- See the ISA for full documentation of everything that will abort transactions.
+
+
+Syscalls
+========
+
+Syscalls made from within an active transaction will not be performed and the
+transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL
+| TM_CAUSE_PERSISTENT.
+
+Syscalls made from within a suspended transaction are performed as normal and
+the transaction is not explicitly doomed by the kernel. However, what the
+kernel does to perform the syscall may result in the transaction being doomed
+by the hardware. The syscall is performed in suspended mode so any side
+effects will be persistent, independent of transaction success or failure. No
+guarantees are provided by the kernel about which syscalls will affect
+transaction success.
+
+Care must be taken when relying on syscalls to abort during active transactions
+if the calls are made via a library. Libraries may cache values (which may
+give the appearance of success) or perform operations that cause transaction
+failure before entering the kernel (which may produce different failure codes).
+Examples are glibc's getpid() and lazy symbol resolution.
+
+
+Signals
+=======
+
+Delivery of signals (both sync and async) during transactions provides a second
+thread state (ucontext/mcontext) to represent the second transactional register
+state. Signal delivery 'treclaim's to capture both register states, so signals
+abort transactions. The usual ucontext_t passed to the signal handler
+represents the checkpointed/original register state; the signal appears to have
+arisen at 'tbegin+4'.
+
+If the sighandler ucontext has uc_link set, a second ucontext has been
+delivered. For future compatibility the MSR.TS field should be checked to
+determine the transactional state -- if so, the second ucontext in uc->uc_link
+represents the active transactional registers at the point of the signal.
+
+For 64-bit processes, uc->uc_mcontext.regs->msr is a full 64-bit MSR and its TS
+field shows the transactional mode.
+
+For 32-bit processes, the mcontext's MSR register is only 32 bits; the top 32
+bits are stored in the MSR of the second ucontext, i.e. in
+uc->uc_link->uc_mcontext.regs->msr. The top word contains the transactional
+state TS.
+
+However, basic signal handlers don't need to be aware of transactions
+and simply returning from the handler will deal with things correctly:
+
+Transaction-aware signal handlers can read the transactional register state
+from the second ucontext. This will be necessary for crash handlers to
+determine, for example, the address of the instruction causing the SIGSEGV.
+
+Example signal handler::
+
+ void crash_handler(int sig, siginfo_t *si, void *uc)
+ {
+ ucontext_t *ucp = uc;
+ ucontext_t *transactional_ucp = ucp->uc_link;
+
+ if (ucp_link) {
+ u64 msr = ucp->uc_mcontext.regs->msr;
+ /* May have transactional ucontext! */
+ #ifndef __powerpc64__
+ msr |= ((u64)transactional_ucp->uc_mcontext.regs->msr) << 32;
+ #endif
+ if (MSR_TM_ACTIVE(msr)) {
+ /* Yes, we crashed during a transaction. Oops. */
+ fprintf(stderr, "Transaction to be restarted at 0x%llx, but "
+ "crashy instruction was at 0x%llx\n",
+ ucp->uc_mcontext.regs->nip,
+ transactional_ucp->uc_mcontext.regs->nip);
+ }
+ }
+
+ fix_the_problem(ucp->dar);
+ }
+
+When in an active transaction that takes a signal, we need to be careful with
+the stack. It's possible that the stack has moved back up after the tbegin.
+The obvious case here is when the tbegin is called inside a function that
+returns before a tend. In this case, the stack is part of the checkpointed
+transactional memory state. If we write over this non transactionally or in
+suspend, we are in trouble because if we get a tm abort, the program counter and
+stack pointer will be back at the tbegin but our in memory stack won't be valid
+anymore.
+
+To avoid this, when taking a signal in an active transaction, we need to use
+the stack pointer from the checkpointed state, rather than the speculated
+state. This ensures that the signal context (written tm suspended) will be
+written below the stack required for the rollback. The transaction is aborted
+because of the treclaim, so any memory written between the tbegin and the
+signal will be rolled back anyway.
+
+For signals taken in non-TM or suspended mode, we use the
+normal/non-checkpointed stack pointer.
+
+Any transaction initiated inside a sighandler and suspended on return
+from the sighandler to the kernel will get reclaimed and discarded.
+
+Failure cause codes used by kernel
+==================================
+
+These are defined in <asm/reg.h>, and distinguish different reasons why the
+kernel aborted a transaction:
+
+ ====================== ================================
+ TM_CAUSE_RESCHED Thread was rescheduled.
+ TM_CAUSE_TLBI Software TLB invalid.
+ TM_CAUSE_FAC_UNAV FP/VEC/VSX unavailable trap.
+ TM_CAUSE_SYSCALL Syscall from active transaction.
+ TM_CAUSE_SIGNAL Signal delivered.
+ TM_CAUSE_MISC Currently unused.
+ TM_CAUSE_ALIGNMENT Alignment fault.
+ TM_CAUSE_EMULATE Emulation that touched memory.
+ ====================== ================================
+
+These can be checked by the user program's abort handler as TEXASR[0:7]. If
+bit 7 is set, it indicates that the error is consider persistent. For example
+a TM_CAUSE_ALIGNMENT will be persistent while a TM_CAUSE_RESCHED will not.
+
+GDB
+===
+
+GDB and ptrace are not currently TM-aware. If one stops during a transaction,
+it looks like the transaction has just started (the checkpointed state is
+presented). The transaction cannot then be continued and will take the failure
+handler route. Furthermore, the transactional 2nd register state will be
+inaccessible. GDB can currently be used on programs using TM, but not sensibly
+in parts within transactions.
+
+POWER9
+======
+
+TM on POWER9 has issues with storing the complete register state. This
+is described in this commit::
+
+ commit 4bb3c7a0208fc13ca70598efd109901a7cd45ae7
+ Author: Paul Mackerras <paulus@ozlabs.org>
+ Date: Wed Mar 21 21:32:01 2018 +1100
+ KVM: PPC: Book3S HV: Work around transactional memory bugs in POWER9
+
+To account for this different POWER9 chips have TM enabled in
+different ways.
+
+On POWER9N DD2.01 and below, TM is disabled. ie
+HWCAP2[PPC_FEATURE2_HTM] is not set.
+
+On POWER9N DD2.1 TM is configured by firmware to always abort a
+transaction when tm suspend occurs. So tsuspend will cause a
+transaction to be aborted and rolled back. Kernel exceptions will also
+cause the transaction to be aborted and rolled back and the exception
+will not occur. If userspace constructs a sigcontext that enables TM
+suspend, the sigcontext will be rejected by the kernel. This mode is
+advertised to users with HWCAP2[PPC_FEATURE2_HTM_NO_SUSPEND] set.
+HWCAP2[PPC_FEATURE2_HTM] is not set in this mode.
+
+On POWER9N DD2.2 and above, KVM and POWERVM emulate TM for guests (as
+described in commit 4bb3c7a0208f), hence TM is enabled for guests
+ie. HWCAP2[PPC_FEATURE2_HTM] is set for guest userspace. Guests that
+makes heavy use of TM suspend (tsuspend or kernel suspend) will result
+in traps into the hypervisor and hence will suffer a performance
+degradation. Host userspace has TM disabled
+ie. HWCAP2[PPC_FEATURE2_HTM] is not set. (although we make enable it
+at some point in the future if we bring the emulation into host
+userspace context switching).
+
+POWER9C DD1.2 and above are only available with POWERVM and hence
+Linux only runs as a guest. On these systems TM is emulated like on
+POWER9N DD2.2.
+
+Guest migration from POWER8 to POWER9 will work with POWER9N DD2.2 and
+POWER9C DD1.2. Since earlier POWER9 processors don't support TM
+emulation, migration from POWER8 to POWER9 is not supported there.