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// 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);
}
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