x86-64: Only set max_pfn_mapped to 512 MiB if we enter via head_64.S

head_64.S maps up to 512 MiB, but that is not necessarity true for
other entry paths, such as Xen.

Thus, co-locate the setting of max_pfn_mapped with the code to
actually set up the page tables in head_64.S. The 32-bit code is
already so co-located. (The Xen code already sets max_pfn_mapped
correctly for its own use case.)

-v2:

Yinghai fixed the following bug in this patch:

|
| max_pfn_mapped is in .bss section, so we need to set that
| after bss get cleared. Without that we crash on bootup.
|
| That is safe because Xen does not call x86_64_start_kernel().
|

Signed-off-by: Jeremy Fitzhardinge <[e-mail]>
Fixed-by: Yinghai Lu <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/head64.c

···
 	/* Cleanup the over mapped high alias */
 	cleanup_highmap();
 
+	max_pfn_mapped = KERNEL_IMAGE_SIZE >> PAGE_SHIFT;
+
 	for (i = 0; i < NUM_EXCEPTION_VECTORS; i++) {
 #ifdef CONFIG_EARLY_PRINTK
 		set_intr_gate(i, &early_idt_handlers[i]);

arch/x86/kernel/setup.c

···
 		max_low_pfn = max_pfn;
 
 	high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
-	max_pfn_mapped = KERNEL_IMAGE_SIZE >> PAGE_SHIFT;
 #endif
 
 	/*