linux/arch/x86/kernel/trampoline.c

#include <linux/io.h>
#include <linux/memblock.h>

#include <asm/trampoline.h>
#include <asm/cacheflush.h>
#include <asm/pgtable.h>

unsigned char *x86_trampoline_base;

void __init setup_trampolines(void)
{
	phys_addr_t mem;
	size_t size = PAGE_ALIGN(x86_trampoline_end - x86_trampoline_start);

	/* Has to be in very low memory so we can execute real-mode AP code. */
	mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
	if (!mem)
		panic("Cannot allocate trampoline\n");

	x86_trampoline_base = __va(mem);
	memblock_reserve(mem, size);

	printk(KERN_DEBUG "Base memory trampoline at [%p] %llx size %zu\n",
	       x86_trampoline_base, (unsigned long long)mem, size);

	memcpy(x86_trampoline_base, x86_trampoline_start, size);
}

/*
 * setup_trampolines() gets called very early, to guarantee the
 * availability of low memory.  This is before the proper kernel page
 * tables are set up, so we cannot set page permissions in that
 * function.  Thus, we use an arch_initcall instead.
 */
static int __init configure_trampolines(void)
{
	size_t size = PAGE_ALIGN(x86_trampoline_end - x86_trampoline_start);

	set_memory_x((unsigned long)x86_trampoline_base, size >> PAGE_SHIFT);
	return 0;
}
arch_initcall(configure_trampolines);