x86/smpboot: Write the top-of-stack block in cpu_smpboot_alloc()

This allows the AP boot assembly use per-cpu variables, and brings the
semantics closer to that of the BSP, which can use per-cpu variables from the
start of day.

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

xen/arch/x86/smpboot.c

@@ -329,7 +329,6 @@ void start_secondary(void *unused)
 
     /* Critical region without IDT or TSS.  Any fault is deadly! */
 
-    set_processor_id(cpu);
     set_current(idle_vcpu[cpu]);
     this_cpu(curr_vcpu) = idle_vcpu[cpu];
     rdmsrl(MSR_EFER, this_cpu(efer));
@@ -986,6 +985,7 @@ static void cpu_smpboot_free(unsigned int cpu, bool remove)
 
 static int cpu_smpboot_alloc(unsigned int cpu)
 {
+    struct cpu_info *info;
     unsigned int i, memflags = 0;
     nodeid_t node = cpu_to_node(cpu);
     seg_desc_t *gdt;
@@ -999,6 +999,11 @@ static int cpu_smpboot_alloc(unsigned int cpu)
         stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, memflags);
     if ( stack_base[cpu] == NULL )
         goto out;
+
+    info = get_cpu_info_from_stack((unsigned long)stack_base[cpu]);
+    info->processor_id = cpu;
+    info->per_cpu_offset = __per_cpu_offset[cpu];
+
     memguard_guard_stack(stack_base[cpu]);
 
     gdt = per_cpu(gdt, cpu) ?: alloc_xenheap_pages(0, memflags);

xen/include/asm-x86/current.h

@@ -100,11 +100,6 @@ static inline struct cpu_info *get_cpu_info(void)
 #define current               (get_current())
 
 #define get_processor_id()    (get_cpu_info()->processor_id)
-#define set_processor_id(id)  do {                                      \
-    struct cpu_info *ci__ = get_cpu_info();                             \
-    ci__->per_cpu_offset = __per_cpu_offset[ci__->processor_id = (id)]; \
-} while (0)
-
 #define guest_cpu_user_regs() (&get_cpu_info()->guest_cpu_user_regs)
 
 /*