powerpc/perf: Fix kernel address leak via sampling registers

powerpc/perf: Fix kernel address leak via sampling registers

Current code in power_pmu_disable() does not clear the sampling
registers like Sampling Instruction Address Register (SIAR) and
Sampling Data Address Register (SDAR) after disabling the PMU. Since
these are userspace readable and could contain kernel addresses, add
code to explicitly clear the content of these registers.

Also add a "context synchronizing instruction" to enforce no further
updates to these registers as suggested by Power ISA v3.0B. From
section 9.4, on page 1108:

  "If an mtspr instruction is executed that changes the value of a
  Performance Monitor register other than SIAR, SDAR, and SIER, the
  change is not guaranteed to have taken effect until after a
  subsequent context synchronizing instruction has been executed (see
  Chapter 11. "Synchronization Requirements for Context Alterations"
  on page 1133)."

Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
[mpe: Massage change log and add ISA reference]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>