x86/boot: attempt to print trace and panic on AP bring up stall

With the current AP bring up code, Xen can get stuck indefinitely if an AP
freezes during boot after the 'callin' step.  Introduce a 5s timeout while
waiting for APs to finish startup.

On failure of an AP to complete startup, send an NMI to trigger the
printing of a stack backtrace on the stuck AP and panic on the BSP.

This patch was done while investigating the issue caused by Intel erratum
ICX143.  It wasn't helpful in that case, but it's still and improvement
when debugging AP bring up related issues.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/smpboot.c

@@ -1375,6 +1375,7 @@ int cpu_add(uint32_t apic_id, uint32_t acpi_id, uint32_t pxm)
 int __cpu_up(unsigned int cpu)
 {
     int apicid, ret;
+    s_time_t start;
 
     if ( (apicid = x86_cpu_to_apicid[cpu]) == BAD_APICID )
         return -ENODEV;
@@ -1393,10 +1394,18 @@ int __cpu_up(unsigned int cpu)
     time_latch_stamps();
 
     set_cpu_state(CPU_STATE_ONLINE);
+    start = NOW();
     while ( !cpu_online(cpu) )
     {
         cpu_relax();
         process_pending_softirqs();
+        if ( (NOW() - start) > SECONDS(5) )
+        {
+            /* AP is stuck, send NMI and panic. */
+            show_execution_state_nmi(cpumask_of(cpu), true);
+            panic("APIC ID %#x (CPU%u) stuck while starting up\n",
+                  apicid, cpu);
+        }
     }
 
     return 0;