[RFC Patch 0/2] PPC64-HWBKPT: Hardware Breakpoint interfaces - ver XV

[K.Prasad]

Hi Ben,
	Please find the version XV of the patch that ports the
hw-breakpoint interfaces (in kernel/hw_breakpoint.c) to PPC64. The new
patch includes the changes as listed below.

In particular, comments are sought about the changes in Kconfig, Makefile
and cputable.h and whether they clearly delimit the changes to the target
architecture (ppc64 server processors with one DABR).

Changelog - ver XV
--------------------
(Version XIV: linuxppc-dev ref: 20100308181232.GA3406 at in.ibm.com)

- 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

Thanks,
K.Prasad

Changelog - ver XIV
--------------------
(Version XIII: linuxppc-dev ref: 20100215055605.GB3670 at in.ibm.com)

- 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: 20100121084640.GA3252 at in.ibm.com)

- 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: 20100119091234.GA9971 at in.ibm.com)

- 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: 20091211160144.GA23156 at in.ibm.com)
- 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