x86/cpu: Rewrite initialize_cpu_data() for clarity

Without passing opinion on the behaviour of this function, it is deceptive to
read (I've twice now mistaken it for resetting boot_cpu_data), and
inefficient.

Instead of having a 256 byte object on the stack and a double copy, copy
boot_cpu_data directly, then reset parts of cpu_data[cpu] in place.  Leave
some comments behind explaining what's happening.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/smpboot.c

@@ -94,12 +94,14 @@ void *stack_base[NR_CPUS];
 
 void initialize_cpu_data(unsigned int cpu)
 {
-    struct cpuinfo_x86 c = boot_cpu_data;
+    struct cpuinfo_x86 *c = &cpu_data[cpu];
 
-    /* Must not partially clear the BSP's collected data. */
+    /* First, inherit from boot_cpu_data */
+    *c = boot_cpu_data;
+
+    /* Second, reset most of it, except if we're the BSP at early boot. */
     if ( cpu || system_state > SYS_STATE_smp_boot )
-        reset_cpuinfo(&c, true);
-    cpu_data[cpu] = c;
+        reset_cpuinfo(c, true);
 }
 
 static bool smp_store_cpu_info(unsigned int id)