linux/arch/x86/boot/compressed/pgtable_64.c

#include <asm/processor.h>
#include "pgtable.h"

/*
 * __force_order is used by special_insns.h asm code to force instruction
 * serialization.
 *
 * It is not referenced from the code, but GCC < 5 with -fPIE would fail
 * due to an undefined symbol. Define it to make these ancient GCCs work.
 */
unsigned long __force_order;

#define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */

struct paging_config {
	unsigned long trampoline_start;
	unsigned long l5_required;
};

/*
 * Trampoline address will be printed by extract_kernel() for debugging
 * purposes.
 *
 * Avoid putting the pointer into .bss as it will be cleared between
 * paging_prepare() and extract_kernel().
 */
unsigned long *trampoline_32bit __section(.data);

struct paging_config paging_prepare(void)
{
	struct paging_config paging_config = {};
	unsigned long bios_start, ebda_start;

	/*
	 * Check if LA57 is desired and supported.
	 *
	 * There are two parts to the check:
	 *   - if the kernel supports 5-level paging: CONFIG_X86_5LEVEL=y
	 *   - if the machine supports 5-level paging:
	 *     + CPUID leaf 7 is supported
	 *     + the leaf has the feature bit set
	 *
	 * That's substitute for boot_cpu_has() in early boot code.
	 */
	if (IS_ENABLED(CONFIG_X86_5LEVEL) &&
			native_cpuid_eax(0) >= 7 &&
			(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
		paging_config.l5_required = 1;
	}

	/*
	 * Find a suitable spot for the trampoline.
	 * This code is based on reserve_bios_regions().
	 */

	ebda_start = *(unsigned short *)0x40e << 4;
	bios_start = *(unsigned short *)0x413 << 10;

	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
		bios_start = BIOS_START_MAX;

	if (ebda_start > BIOS_START_MIN && ebda_start < bios_start)
		bios_start = ebda_start;

	/* Place the trampoline just below the end of low memory, aligned to 4k */
	paging_config.trampoline_start = bios_start - TRAMPOLINE_32BIT_SIZE;
	paging_config.trampoline_start = round_down(paging_config.trampoline_start, PAGE_SIZE);

	trampoline_32bit = (unsigned long *)paging_config.trampoline_start;

	return paging_config;
}