tree ca43baa684b371ee00cf93c5ee8268507bc7137c
parent dddd3379a619a4cb8247bfd3c94ca9ae3797aa2e
author Don Zickus <dzickus@redhat.com> 1290462923 -0500
committer Ingo Molnar <mingo@elte.hu> 1290780057 +0100

x86, perf, nmi: Disable perf if counters are not accessible

In a kvm virt guests, the perf counters are not emulated.  Instead they
return zero on a rdmsrl. The perf nmi handler uses the fact that crossing
a zero means the counter overflowed (for those counters that do not have
specific interrupt bits). Therefore on kvm guests, perf will swallow all
NMIs thinking the counters overflowed.

This causes problems for subsystems like kgdb which needs NMIs to do its
magic. This problem was discovered by running kgdb tests.

The solution is to write garbage into a perf counter during the
initialization and hopefully reading back the same number.  On kvm
guests, the value will be read back as zero and we disable perf as
a result.

Reported-by: Jason Wessel <jason.wessel@windriver.com>
Patch-inspired-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <1290462923-30734-1-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>