x86/boot: Annotate the Real Mode entry points

... because its already hard enough to follow.  Cross reference the locations
in C which set the entrypoints up, and state the alignment requirements and
entry conditions.

Drop a redundant .align 16, and panic() in do_boot_cpu() if the AP trampoline
isn't set up properly rather than blindly continuing and letting the APs
execute junk, or shifting part of the address into unrelated fields in ICR.

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

xen/arch/x86/boot/trampoline.S

@@ -38,6 +38,13 @@
 
         .code16
 
+/*
+ * do_boot_cpu() programs the Startup-IPI to point here.  Due to the SIPI
+ * format, the relocated entrypoint must be 4k aligned.
+ *
+ * It is entered in Real Mode, with %cs = trampoline_realmode_entry >> 4 and
+ * %ip = 0.
+ */
 GLOBAL(trampoline_realmode_entry)
         mov     %cs,%ax
         mov     %ax,%ds

xen/arch/x86/boot/wakeup.S

@@ -2,7 +2,15 @@
 
 #define wakesym(sym) (sym - wakeup_start)
 
-        .align 16
+/*
+ * acpi_sleep_prepare() programs the S3 wakeup vector to point here.
+ *
+ * The ACPI spec says that we shall be entered in Real Mode with:
+ *   %cs = wakeup_start >> 4
+ *   %ip = wakeup_start & 0xf
+ *
+ * As wakeup_start is 16-byte aligned, %ip is 0 in practice.
+ */
 ENTRY(wakeup_start)
         cli
         cld

xen/arch/x86/smpboot.c

@@ -548,9 +548,12 @@ static int do_boot_cpu(int apicid, int cpu)
 
     booting_cpu = cpu;
 
-    /* start_eip had better be page-aligned! */
     start_eip = setup_trampoline();
 
+    /* start_eip needs be page aligned, and below the 1M boundary. */
+    if ( start_eip & ~0xff000 )
+        panic("AP trampoline %#lx not suitably positioned\n", start_eip);
+
     /* So we see what's up   */
     if ( opt_cpu_info )
         printk("Booting processor %d/%d eip %lx\n",