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[Xen-devel] [PATCH] x86/nmi: correctly check MSB of P6 performance counter MSR in watchdog

The logic currently tries to work out if a recent overflow (that indicates
that NMI comes from the watchdog) happened by checking MSB of performance
counter MSR that is initially sign extended from a negative value
that we program it to. A possibly incorrect assumption here is that
MSB is always bit 32 while on modern hardware it's usually 47 and
the actual bit-width is reported through CPUID. Checking bit 32 for
overflows is usually fine since we never program it to anything
exceeding 32-bits and NMI is handled shortly after overflow occurs.

A problematic scenario that we saw occurs on systems where SMIs taking
significant time are possible. In that case, NMI handling is deferred to
the point firmware exits SMI which might take enough time for the counter
to go through bit 32 and set it to 1 again. So the logic described above
will misread it and report an unknown NMI erroneously.

Fortunately, we can use the actual MSB, which is usually higher than the
currently hardcoded 32, and treat this case correctly at least on modern

Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
 xen/arch/x86/nmi.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/nmi.c b/xen/arch/x86/nmi.c
index bfe9777..6adb08a 100644
--- a/xen/arch/x86/nmi.c
+++ b/xen/arch/x86/nmi.c
@@ -37,6 +37,7 @@ unsigned int nmi_watchdog = NMI_NONE;
 static unsigned int nmi_hz = HZ;
 static unsigned int nmi_perfctr_msr;   /* the MSR to reset in NMI handler */
 static unsigned int nmi_p4_cccr_val;
+static unsigned int nmi_p6_event_width;
 static DEFINE_PER_CPU(struct timer, nmi_timer);
 static DEFINE_PER_CPU(unsigned int, nmi_timer_ticks);
@@ -123,7 +124,8 @@ int nmi_active;
 #define P6_EVNTSEL_USR         (1 << 16)
-#define P6_EVENT_WIDTH          32
+/* Bit width of IA32_PMCx MSRs is reported using CPUID.0AH:EAX[23:16]. */
+#define P6_EVENT_WIDTH_MASK    (((1 << 8) - 1) << 16)
 #define P4_ESCR_EVENT_SELECT(N)        ((N)<<25)
 #define P4_CCCR_OVF_PMI0       (1<<26)
@@ -323,6 +325,7 @@ static void setup_p6_watchdog(unsigned counter)
     unsigned int evntsel;
     nmi_perfctr_msr = MSR_P6_PERFCTR(0);
+    nmi_p6_event_width = MASK_EXTR(cpuid_eax(0xa), P6_EVENT_WIDTH_MASK);
     clear_msr_range(MSR_P6_EVNTSEL(0), 2);
     clear_msr_range(MSR_P6_PERFCTR(0), 2);
@@ -529,7 +532,7 @@ bool nmi_watchdog_tick(const struct cpu_user_regs *regs)
         else if ( nmi_perfctr_msr == MSR_P6_PERFCTR(0) )
             rdmsrl(MSR_P6_PERFCTR(0), msr_content);
-            if ( msr_content & (1ULL << P6_EVENT_WIDTH) )
+            if ( msr_content & (1ULL << (nmi_p6_event_width - 1)) )
                 watchdog_tick = false;

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