// SPDX-License-Identifier: GPL-2.0#include <linux/kernel.h>#include <linux/init.h>#include <linux/memblock.h> #include <asm/setup.h>#include <asm/bios_ebda.h> /* * This function reserves all conventional PC system BIOS related * firmware memory areas (some of which are data, some of which * are code), that must not be used by the kernel as available * RAM. * * The BIOS places the EBDA/XBDA at the top of conventional * memory, and usually decreases the reported amount of * conventional memory (int 0x12) too. * * This means that as a first approximation on most systems we can * guess the reserved BIOS area by looking at the low BIOS RAM size * value and assume that everything above that value (up to 1MB) is * reserved. * * But life in firmware country is not that simple: * * - This code also contains a quirk for Dell systems that neglect *   to reserve the EBDA area in the 'RAM size' value ... * * - The same quirk also avoids a problem with the AMD768MPX *   chipset: reserve a page before VGA to prevent PCI prefetch *   into it (errata #56). (Usually the page is reserved anyways, *   unless you have no PS/2 mouse plugged in.) * * - Plus paravirt systems don't have a reliable value in the *   'BIOS RAM size' pointer we can rely on, so we must quirk *   them too. * * Due to those various problems this function is deliberately * very conservative and tries to err on the side of reserving * too much, to not risk reserving too little. * * Losing a small amount of memory in the bottom megabyte is * rarely a problem, as long as we have enough memory to install * the SMP bootup trampoline which *must* be in this area. * * Using memory that is in use by the BIOS or by some DMA device * the BIOS didn't shut down *is* a big problem to the kernel, * obviously. */ #define BIOS_RAM_SIZE_KB_PTR	0x413 #define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */ void __init reserve_bios_regions(void){	unsigned int bios_start, ebda_start; 	/*	 * NOTE: In a paravirtual environment the BIOS reserved	 * area is absent. We'll just have to assume that the	 * paravirt case can handle memory setup correctly,	 * without our help.	 */	if (!x86_platform.legacy.reserve_bios_regions)		return; 	/*	 * BIOS RAM size is encoded in kilobytes, convert it	 * to bytes to get a first guess at where the BIOS	 * firmware area starts:	 */	bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);	bios_start <<= 10; 	/*	 * If bios_start is less than 128K, assume it is bogus	 * and bump it up to 640K.  Similarly, if bios_start is above 640K,	 * don't trust it.	 */	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)		bios_start = BIOS_START_MAX; 	/* Get the start address of the EBDA page: */	ebda_start = get_bios_ebda(); 	/*	 * If the EBDA start address is sane and is below the BIOS region,	 * then also reserve everything from the EBDA start address up to	 * the BIOS region.	 */	if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)		bios_start = ebda_start; 	/* Reserve all memory between bios_start and the 1MB mark: */	memblock_reserve(bios_start, 0x100000 - bios_start);}