[PATCH] x86/nmi: Fix mis-classification of watchdog NMIs

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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 caluclated propely.

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

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 (mis)classifed 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: 7126b7f806d5 ("x86/CPU: re-work populating of cpu_data[]")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
CC: Teddy Astie <teddy.astie@xxxxxxxxxx>

Found on a system where:

   [root@box ~]# time xen-ucode ./blob

   real    0m9.166s
   user    0m0.001s
   sys     0m9.165s

is changing several expectations, and spurious crashes from mis-classified
watchdog NMIs is just one part of the problem.

I hate this fix, but it's the only thing which I consider remotely safe to
backport.  Recent attempts to alter CPUID ordering have 0 success at being
bug-free.

I have not investigated what else was broken by the cpu_data[] change, owing
to a lack of time on my part.  I would be amazed if this is the only thing.
---
 xen/arch/x86/cpu/vpmu_intel.c | 2 +-
 xen/arch/x86/nmi.c            | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/vpmu_intel.c b/xen/arch/x86/cpu/vpmu_intel.c
index ed9f62b9366d..af6cb0a85cdd 100644
--- a/xen/arch/x86/cpu/vpmu_intel.c
+++ b/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 ( bsp_cpu_data.cpuid_level >= 0xa )
         version = MASK_EXTR(cpuid_eax(0xa), PMU_VERSION_MASK);
 
     switch ( version )
diff --git a/xen/arch/x86/nmi.c b/xen/arch/x86/nmi.c
index 91f95fe6d080..ec85516609b0 100644
--- a/xen/arch/x86/nmi.c
+++ b/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;
-- 
2.39.5