Commit 242214f9 authored by Don Zickus's avatar Don Zickus Committed by Ingo Molnar

perf, x86: P4 PMU - Read proper MSR register to catch unflagged overflows

The read of a proper MSR register was missed and instead of
counter the configration register was tested (it has
ARCH_P4_UNFLAGGED_BIT always cleared) leading to unknown NMI
hitting the system. As result the user may obtain "Dazed and
confused, but trying to continue" message. Fix it by reading a
proper MSR register.

When an NMI happens on a P4, the perf nmi handler checks the
configuration register to see if the overflow bit is set or not
before taking appropriate action.  Unfortunately, various P4
machines had a broken overflow bit, so a backup mechanism was
implemented.  This mechanism checked to see if the counter
rolled over or not.

A previous commit that implemented this backup mechanism was
broken. Instead of reading the counter register, it used the
configuration register to determine if the counter rolled over
or not. Reading that bit would give incorrect results.

This would lead to 'Dazed and confused' messages for the end
user when using the perf tool (or if the nmi watchdog is

The fix is to read the counter register before determining if
the counter rolled over or not.
Signed-off-by: default avatarDon Zickus <>
Signed-off-by: default avatarCyrill Gorcunov <>
Cc: Lin Ming <>
LKML-Reference: <>
Signed-off-by: default avatarIngo Molnar <>
parent 60e4b10c
......@@ -777,6 +777,7 @@ static inline int p4_pmu_clear_cccr_ovf(struct hw_perf_event *hwc)
* the counter has reached zero value and continued counting before
* real NMI signal was received:
rdmsrl(hwc->event_base, v);
return 1;
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment