Intel x86 family 45 support of btrace

[Doug Evans <dje at google dot com>]

Hi.

I'm playing with btrace on my machine.
It has CPU model 6, family 45, which according to
nat/linux-btrace.c:intel_supports_bts
does not support it.

I changed the function to say bts is supported anyway and got
the tests in gdb.btrace/*.exp that previously weren't being run
to run, and all pass except for one.

FAIL: gdb.btrace/exception.exp: indented (pattern 1)

I haven't dug into why this fails, but a lot seems to be working.

The log entry for when this check was added says:

---snip---
date: 2013/03/11 08:38:27;  author: mmetzger;  state: Exp;  lines: +186 -1
LBR, BTM, or BTS records may have incorrect branch "from" information afer  
an
EIST transition, T-states, C1E, or Adaptive Thermal Throttling (AAJ122).

This results in sporadic test fails. Disable btrace on those processors.
---snip---

I'm wondering how useful this feature is even if it's not always correct.
E.g., if it is usually correct, would the user still find the feature useful
enough to cope with the shortcomings?
IOW, would it make sense to add the ability to turn on the feature
anyways (defaulting to "off" of course) ?