PPC64-HWBKPT: Hardware Breakpoint interfaces - ver XXIII

Hi All,
Please find a new version of the patchset that implement hardware
breakpoint interfaces for the PowerPC BookIII S processor. The changes are few and as described below.

Changelog - ver XXIII
--------------------
(Version XXII: [email protected])
- Detection of extraneous breakpoint exceptions is now done using a boolean flag in 'struct arch_hw_breakpoint'.
- A dangling put_cpu() (remnant from previous patch versions) in arch_unregister_hw_breakpoint() is now removed.

Kindly let me know your comments.

Thanks,
K.Prasad

Changelog - ver XXII
--------------------
(Version XXI: linuxppc-dev ref:[email protected])
- Extraneous breakpoint exceptions are now properly handled; causative instruction will be single-stepped and debug register values restored.
- Restoration of breakpoints during signal handling through thread_change_pc() had defects which are now fixed.
- Breakpoints are flushed through flush_ptrace_hw_breakpoint() call in both flush_thread() and prepare_to_copy() functions. 'ptrace_bps[]' and 'last_hit_ubp' members are now promptly cleaned-up.
- Single-step exception is now conditionally emulated upon hitting alignment_exception.
- Rebased to commit 31f46717997a83bdf6db0dd04810c0a329eb3148 of linux-2.6 tree.

Changelog - ver XXI
--------------------
(Version XX: linuxppc-dev ref:[email protected])
- Decision to emulate an instruction is now based on whether the causative instruction is in user_mode() or not.
- Breakpoints don't have to be cleared during sigreturn. A 'double-hit' on hw_breakpoint_handler() is harmless for non-ptrace instructions.
- Minor changes to aid code brevity.

Changelog - ver XX
--------------------
(Version XIX: linuxppc-dev ref: [email protected])
- Non task-bound breakpoints will only be emulated. Breakpoint will be unregistered with a warning if emulation fails.

Changelog - ver XIX
--------------------
(Version XVIII: linuxppc-dev ref: [email protected])
- Increased coverage of breakpoints during concurrent alignment_exception and signal handling (which previously had 'blind-spots').
- Support for kernel-thread breakpoints and kernel-space breakpoints inside the context of a user-space process.
- Patches re-based to commit f4b87dee923342505e1ddba8d34ce9de33e75050, thereby necessitating minor changes to arch_validate_hwbkpt_settings().

Changelog - ver XVIII
--------------------
(Version XVII: linuxppc-dev ref: [email protected])
- Slight code restructuring for brevity and coding-style corrections.
- emulate_single_step() now notifies DIE_SSTEP to registered handlers; causes single_step_dabr_instruction() to be invoked after alignment_exception.
- hw-breakpoint restoration variables are cleaned-up before unregistration through arch_unregister_hw_breakpoint().
- SIGTRAP is no longer generated for non-ptrace user-space breakpoints.

Changelog - ver XVII
--------------------
(Version XVI: linuxppc-dev ref: [email protected])
- CONFIG_HAVE_HW_BREAKPOINT is now used to define the scope of the new code (in lieu of CONFIG_PPC_BOOK3S_64).
- CONFIG_HAVE_HW_BREAKPOINT is now dependant upon CONFIG_PERF_EVENTS and CONFIG_PPC_BOOK3S_64 (to overcome build failures under certain configs).
- Included a target in arch/powerpc/lib/Makefile to build sstep.o when HAVE_HW_BREAKPOINT.
- Added a dummy definition for hw_breakpoint_disable() when !HAVE_HW_BREAKPOINT.
- Tested builds under defconfigs for ppc64, cell and g5 (found no patch induced failures).

Changelog - ver XVI
--------------------
(Version XV: linuxppc-dev ref: [email protected])
- Used a new config option CONFIG_PPC_BOOK3S_64 (in lieu of CONFIG_PPC64/CPU_FTR_HAS_DABR) to limit the scope of the new code.
- Disabled breakpoints before kexec of the machine using hw_breakpoint_disable().
- Minor optimisation in exception-64s.S to check for data breakpoint exceptions in DSISR finally (after check for other causes) + changes in code comments and representation of DSISR_DABRMATCH constant.
- Rebased to commit ae6be51ed01d6c4aaf249a207b4434bc7785853b of linux-2.6.

Changelog - ver XV
--------------------
(Version XIV: linuxppc-dev ref: [email protected])

- Additional patch to disable interrupts during data breakpoint exception handling.
- Moved HBP_NUM definition to cputable.h under a new CPU_FTR definition (CPU_FTR_HAS_DABR).
- Filtering of extraneous exceptions (due to accesses outside symbol length) is by-passed for ptrace requests.
- Removed flush_ptrace_hw_breakpoint() from __switch_to() due to incorrect coding placement.
- Changes to code comments as per community reviews for previous version.
- Minor coding-style changes in hw_breakpoint.c as per review comments.
- Re-based to commit ae6be51ed01d6c4aaf249a207b4434bc7785853b of linux-2.6

Changelog - ver XIV
--------------------
(Version XIII: linuxppc-dev ref: [email protected])

- Removed the 'name' field from 'struct arch_hw_breakpoint'.
- All callback invocations through bp->overflow_handler() are replaced with perf_bp_event().
- Removed the check for pre-existing single-stepping mode in hw_breakpoint_handler() as this check is unreliable while in kernel-space. Side effect of this change is the non-triggering of hw-breakpoints while single-stepping kernel through KGDB or Xmon.
- Minor code-cleanups and addition of comments in hw_breakpoint_handler() and single_step_dabr_instruction().
- Re-based to commit 25cf84cf377c0aae5dbcf937ea89bc7893db5176 of linux-2.6

Changelog - ver XIII
--------------------
(Version XII: linuxppc-dev ref: [email protected])

- Fixed a bug for user-space breakpoints (triggered following the failure of a breakpoint request).
- Re-based on commit 724e6d3fe8003c3f60bf404bf22e4e331327c596 of linux-2.6

Changelog - ver XII
--------------------
(Version XI: linuxppc-dev ref: [email protected])

- Unset MSR_SE only if kernel was not previously in single-step mode.
- Pre-emption is now enabled before returning from the hw-breakpoint exception handler.
- Variables to track the source of single-step exception (breakpoint from kernel, user-space vs single-stepping due to other requests) are added.
- Extraneous hw-breakpoint exceptions (due to memory accesses lying outside monitored symbol length) is now done for both kernel and user-space (previously only user-space).
- single_step_dabr_instruction() now returns NOTIFY_DONE if kernel was in single-step mode even before the hw-breakpoint. This enables other users of single-step mode to be notified of the exception.
- User-space instructions are not emulated from kernel-space, they are instead single-stepped.

Changelog - ver XI
------------------
(Version X: linuxppc-dev ref: [email protected])
- Conditionally unset MSR_SE in the single-step handler
- Added comments to explain the duration and need for pre-emption disable following hw-breakpoint exception.

Changelog - ver X
------------------
- Re-write the PPC64 patches for the new implementation of hw-breakpoints that uses the perf-layer.
- Rebased to commit 7622fc234190a37d4e9fe3ed944a2b61a63fca03 of -tip.

Changelog - ver IX
-------------------
- Invocation of user-defined callback will be 'trigger-after-execute' (except for ptrace).
- Creation of a new global per-CPU breakpoint structure to help invocation of user-defined callback from single-step handler.
(Changes made now)
- Validation before registration will fail only if the address does not match the kernel symbol's (if specified) resolved address
(through kallsyms_lookup_name()).
- 'symbolsize' value is expected to within the range contained by the symbol's starting address and the end of a double-word boundary (8 Bytes).
- PPC64's arch-dependant code is now aware of 'cpumask' in 'struct hw_breakpoint' and can accomodate requests for a subset of CPUs in the system.
- Introduced arch_disable_hw_breakpoint() required for <enable><disable>_hw_breakpoint() APIs.

Changelog - ver VIII
-------------------
- Reverting changes to allow one-shot breakpoints only for ptrace requests.
- Minor changes in sanity checking in arch_validate_hwbkpt_settings().
- put_cpu_no_resched() is no longer available. Converted to put_cpu().

Changelog - ver VII
-------------------
- Allow the one-shot behaviour for exception handlers to be defined by the user. A new 'is_one_shot' flag is added to 'struct arch_hw_breakpoint'.

Changelog - ver VI
------------------
The task of identifying 'genuine' breakpoint exceptions from those caused by 'out-of-range' accesses turned out to be more tricky than originally thought. Some changes to this effect were made in version IV of this patchset, but they were not sufficient for user-space. Basically the breakpoint address received through ptrace is always aligned to 8-bytes since ptrace receives an encoded 'data' (consisting of address | translation_enable | bkpt_type), and the size of the symbol is not known. However for kernel-space addresses, the symbol-size can be determined using kallsyms_lookup_size_offset() and this is used to check if DAR (in the exception context) is
'bkpt_address <= DAR <= (bkpt_address + symbol_size)', failing which we conclude it as a stray exception.

The following changes are made to enable check:
- Addition of a symbolsize field in 'struct arch_hw_breakpoint' field.
- Store the size of the 'watched' kernel symbol into 'symbolsize' field in arch_store_info(0 routine.
- Verify if the above described condition is true when is_one_shot is FALSE in hw_breakpoint_handler().

Changelog - ver V
------------------
- Breakpoint requests from ptrace (for user-space) are designed to be one-shot in PPC64. The patch contains changes to retain this behaviour by returning early in hw_breakpoint_handler() [without re-initialising DABR] and unregistering the user-space request in ptrace_triggered(). It is safe to make a unregister_user_hw_breakpoint() call from the breakpoint exception context [through ptrace_triggered()] without giving rise to circular locking-dependancy. This is because there can be no kernel code running on the CPU (which received the exception) with the same spinlock held.

- Minor change in 'type' member of 'struct arch_hw_breakpoint' from u8 to 'int'.

Changelog - ver IV
------------------
- While DABR register requires double-word (8 bytes) aligned addresses, i.e. the breakpoint is active over a range of 8 bytes, PPC64 allows byte-level addressability. This may lead to stray exceptions which have to be ignored in hw_breakpoint_handler(), when DAR != (Breakpoint request address). However DABR will be populated with the requested breakpoint address aligned to the previous double-word address. The code is now modified to store user-requested address in 'bp->info.address' but update the DABR with a double-word aligned address.
- Please note that the Data Breakpoint facility in Xmon is broken as of 2.6.29 and the same has not been integrated into this facility as described in Ver I.

Changelog - ver III
------------------
- Patches are based on commit 08f16e060bf54bdc34f800ed8b5362cdeda75d8b of -tip tree.
- The declarations in arch/powerpc/include/asm/hw_breakpoint.h are done only if CONFIG_PPC64 is defined. This eliminates the need to conditionally include this header file.
- load_debug_registers() is done in start_secondary() i.e. during CPU initialisation.
- arch_check_va_<> routines in hw_breakpoint.c are now replaced with a much simpler is_kernel_addr() check in arch_validate_hwbkpt_settings()
- Return code of hw_breakpoint_handler() when triggered due to Lazy debug register switching is now changed to NOTIFY_STOP.
- The ptrace code no longer sets the TIF_DEBUG task flag as it is proposed to be done in register_user_hw_breakpoint() routine.
- hw_breakpoint_handler() is now modified to use hbp_kernel_pos value to determine if the trigger was a user/kernel space address. The DAR register value is checked with the address stored in 'struct hw_breakpoint' to avoid handling of exceptions that belong to kprobe/Xmon.

Changelog - ver II
------------------
- Split the monolithic patch into six logical patches
- Changed the signature of arch_check_va_in_<user><kernel>space functions. They are now marked static.
- HB_NUM is now called as HBP_NUM (to preserve a consistent short-name convention)
- Introduced hw_breakpoint_disable() and changes to kexec code to disable breakpoints before a reboot.
- Minor changes in ptrace code to use macro-defined constants instead of numbers.
- Introduced a new constant definition INSTRUCTION_LEN in reg.h