Subject: [Patch 0/5] PPC64-HWBKPT: Hardware Breakpoint interfaces - ver XXIV

From: "K.Prasad" <[email-masked]>
Date: Tue, 15 Jun 2010 11:34:14 +0530

Hi Paul,
	Please find a new set of patches with changes as described below.
The patches have been tested for kernel- and user-mode breakpoints (after
applying the two patches you sent, refer message-id:
[email-masked] and message-id:
20100615015459.GA30479@drongo) and are found to work fine!

Changelog - ver XXIV
--------------------
(Version XXII: [email-masked])
- A lockdep issue seen due to invocation of unregister_hw_breakpoint() in
  exception context is now fixed (perf_event_disable() is used instead).
- Fixed improper handling of extraneous exceptions (through clearing of
  extraneous exception flag and preventing the invocation of callback).

Kindly let me know if you have any further comments.

Thanks,
K.Prasad

Changelog - ver XXIII
--------------------
(Version XXII: [email-masked])
- 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.

Changelog - ver XXII
--------------------
(Version XXI: linuxppc-dev ref:[email-masked])
- 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-masked])
- 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-masked])
- 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-masked])
- 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-masked])
- 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-masked])
- 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-masked])
- 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-masked])

- 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-masked])

- 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-masked])

- 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-masked])

- 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-masked])
- 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