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