Subject: [Patch 0/1] PPC64-HWBKPT: Hardware Breakpoint interfaces - ver XIV

From: "K.Prasad" <[email-masked]>
Date: Mon, 8 Mar 2010 23:42:32 +0530

Hi Ben,
        Please find the version XIV 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.

Kindly include this patch as a part of powerpc -next tree to enable wider
testing and to be eventually pushed upstream.

Changelog - ver XIV
--------------------
(Version XIII: linuxppc-dev message-id: [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

Thanks,
K.Prasad

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) causing data exception (due to access
  of NULL/spurious address).
- 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