x86: disable X86_PTRACE_BTS

there's a new ptrace arch level feature in .28:

  config X86_PTRACE_BTS
  bool "Branch Trace Store"

it has broken fork() handling: the old DS area gets copied over into
a new task without clearing it.

Fixes exist but they came too late:

  c5dee61: x86, bts: memory accounting
  bf53de9: x86, bts: add fork and exit handling

and are queued up for v2.6.29. This shows that the facility is still not
tested well enough to release into a stable kernel - disable it for now and
reactivate in .29. In .29 the hardware-branch-tracer will use the DS/BTS
facilities too - hopefully resulting in better code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index b815664..8e99073 100644 (file)
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -520,6 +520,7 @@ config X86_PTRACE_BTS
        bool "Branch Trace Store"
        default y
        depends on X86_DEBUGCTLMSR
+       depends on BROKEN
        help
          This adds a ptrace interface to the hardware's branch trace store.