Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift

[Cyrill Gorcunov]

On Thu, Nov 18, 2010 at 02:32:47PM -0500, Don Zickus wrote:
... 
> On a side note I think I have a fix for the p4 problem but will probably
> need Cyril to look at it.  Basically in, p4_pmu_clear_cccr_ovf() it is
> using the high part of the cccr register to determine if the counter
> overflowed, when it probably wants to use the low bits of the cccr
> register and high bits of the event_base.
>

Thanks a hige Don for pointing to the problem. Here is the patch.
 
  Cyrill
---
perf, x86: P4 PMU - Fix unflagged overflows handling

Jason pointed out that kgdb no longer works with new
nmi-watchdog. Don found the reason -- P4 PMU reads CCCR
register instead of counter itself, it forces NMIs to
be eaten by perf subsystem.

Fix it by reading a proper register.

Reported-by: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Reported-by: Don Zickus <dzickus@xxxxxxxxxx>
Tested-by: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event_p4.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

Index: linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event_p4.c
+++ linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
@@ -753,19 +753,22 @@ out:
 
 static inline int p4_pmu_clear_cccr_ovf(struct hw_perf_event *hwc)
 {
-	int overflow = 0;
-	u32 low, high;
+	u32 overflow = 0;
+	u32 low, low_cccr, high;
 
-	rdmsr(hwc->config_base + hwc->idx, low, high);
+	/* an official way for overflow indication */
+	rdmsr(hwc->config_base + hwc->idx, low_cccr, high);
+	overflow |= (low_cccr & P4_CCCR_OVF);
+
+	/* unflagged overflows */
+	rdmsr(hwc->event_base + hwc->idx, low, high);
+	overflow |= high & 0x80000000;
 
-	/* we need to check high bit for unflagged overflows */
-	if ((low & P4_CCCR_OVF) || !(high & (1 << 31))) {
-		overflow = 1;
+	if (overflow)
 		(void)checking_wrmsrl(hwc->config_base + hwc->idx,
-			((u64)low) & ~P4_CCCR_OVF);
-	}
+			((u64)low_cccr) & ~P4_CCCR_OVF);
 
-	return overflow;
+	return overflow > 0;
 }
 
 static void p4_pmu_disable_pebs(void)
--
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