x86/nmi: Fix mis-classification of watchdog NMIs

It used to be the case that cpu_data[] inherited the BSP's cpuid_level until
the AP had calculated it itself.  Following the rework, cpuid_level has a
placeholder 1 until it is calculated properly.

setup_apic_nmi_watchdog() happens to be called on the BSP after SMP bring-up,
meaning that the first call is on CPU1.  It is also positioned in the window
where cpu_data[] is wrong.

As a result, setup_p6_watchdog()'s one-time calculation of the performance
counter width falls back into Pentium compatibility mode assuming 32bit
counters.  This causes a watchdog NMI which is delayed a little (e.g. from an
SMI), to appear as if it hadn't overflowed, and therefore be considered as not
a watchdog NMI.  On systems where unknown NMIs are treated as fatal, this
results in a spurious crash.

Switch setup_p6_watchdog() to use boot_cpu_data.cpuid_level, which is how this
is checked almost everywhere else.

core2_vpmu_init() used the same pattern to look at leaf 0xa.  Despite being
init code and only running on the BSP, {boot,current}_cpu_data are different
objects, so switch it over to checking boot_cpu_data.cpuid_level too.

Fixes: 7126b7f806 ("x86/CPU: re-work populating of cpu_data[]")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: dc5a77c6c4
master date: 2026-08-17 18:46:08 +0100

xen/arch/x86/cpu/vpmu_intel.c

@@ -896,7 +896,7 @@ const struct arch_vpmu_ops *__init core2_vpmu_init(void)
     unsigned int version = 0;
     unsigned int i;
 
-    if ( current_cpu_data.cpuid_level >= 0xa )
+    if ( boot_cpu_data.cpuid_level >= 0xa )
         version = MASK_EXTR(cpuid_eax(0xa), PMU_VERSION_MASK);
 
     switch ( version )

xen/arch/x86/nmi.c

@@ -321,7 +321,7 @@ static void setup_p6_watchdog(unsigned counter)
 {
     unsigned int evntsel;
 
-    if ( !nmi_p6_event_width && current_cpu_data.cpuid_level >= 0xa )
+    if ( !nmi_p6_event_width && boot_cpu_data.cpuid_level >= 0xa )
         nmi_p6_event_width = MASK_EXTR(cpuid_eax(0xa), P6_EVENT_WIDTH_MASK);
     if ( !nmi_p6_event_width )
         nmi_p6_event_width = P6_EVENT_WIDTH_MIN;