[Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware Breakpoint requests

["K.Prasad" <prasad@linux.vnet.ibm.com>]

Hi All,
	Please find a patch that enables kernel-space breakpoints to be
requested for a subset of the available CPUs in the system. This allows
per-CPU breakpoints and comes with the associated benefit of reduced
overhead during (un)registration.

This enhancement allows exploitation of hardware breakpoint registers by
'perf' which produces a CPU-wise information.

Design changes
--------------
- Every breakpoint request 'consumes' the first available debug register
  (starting from HBP_NUM) in each CPU represented by 'cpumask' field in
  'struct hw_breakpoint'.

- 'hbp_kernel_pos' (that separates kernel-space breakpoints from the
  free/user-space breakpoints) now points to the maximum of debug
  registers consumed on any given CPU.
  -- 'hbp_kernel_pos' is decremented (one-at-a-time) to allow a new-slot
     for kernel-space requests iff all debug registers on the given CPU
     (from HBP_NUM - 1 to 'hbp_kernel_pos' are already consumed.
  -- 'hbp_kernel_pos' is incremented (one-at-a-time) to free a slot iff
     a removal request results in the release of a bkpt request that
     consumed maximum debug registers for kernel-space.

- Every removal request results in compaction of breakpoint registers
  (on a per-cpu basis) to occupy the vacant debug register.
 
The patch is based on commit b6c720b811aed0eeda89f277f13c1bd1bdf721fd of
-tip tree and has been tested to work fine on an x86 machine for both
cases (i.e. system-wide kernel breakpoints and bkpts for a subset of CPUs).

Please let me know your comments on the same.

Thanks,
K.Prasad