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Diffstat (limited to '')
| -rw-r--r-- | arch/x86/kernel/ebda.c | 98 |
1 files changed, 98 insertions, 0 deletions
diff --git a/arch/x86/kernel/ebda.c b/arch/x86/kernel/ebda.c new file mode 100644 index 000000000..38e7d597b --- /dev/null +++ b/arch/x86/kernel/ebda.c @@ -0,0 +1,98 @@ +// 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); +} |