The Intel LBR on some recent processor is capable
of filtering branches by type. The filter is configurable
via the LBR_SELECT MSR register.

There are limitation on how this register can be used.

On Nehalem/Westmere, the LBR_SELECT is shared by the two HT threads
when HT is on. It is private to each core when HT is off.

On SandyBridge, the LBR_SELECT register is private to each thread
when HT is on. It is private to each core when HT is off.

The kernel must manage the sharing of LBR_SELECT. It allows
multiple users on the same logical CPU to use LBR_SELECT as
long as they program it with the same value. Across sibling
CPUs (HT threads), the same restriction applies on NHM/WSM.

This patch implements this sharing logic by leveraging the
mechanism put in place for managing the offcore_response
shared MSR.

We modify __intel_shared_reg_get_constraints() to cause
x86_get_event_constraint() to be called because LBR may
be associated with events that may be counter constrained.

Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1328826068-11713-4-git-send-email-eranian@google.com


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