cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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ebda.c (3259B)

      1// SPDX-License-Identifier: GPL-2.0
      2#include <linux/kernel.h>
      3#include <linux/init.h>
      4#include <linux/memblock.h>
      5
      6#include <asm/setup.h>
      7#include <asm/bios_ebda.h>
      8
      9/*
     10 * This function reserves all conventional PC system BIOS related
     11 * firmware memory areas (some of which are data, some of which
     12 * are code), that must not be used by the kernel as available
     13 * RAM.
     14 *
     15 * The BIOS places the EBDA/XBDA at the top of conventional
     16 * memory, and usually decreases the reported amount of
     17 * conventional memory (int 0x12) too.
     18 *
     19 * This means that as a first approximation on most systems we can
     20 * guess the reserved BIOS area by looking at the low BIOS RAM size
     21 * value and assume that everything above that value (up to 1MB) is
     22 * reserved.
     23 *
     24 * But life in firmware country is not that simple:
     25 *
     26 * - This code also contains a quirk for Dell systems that neglect
     27 *   to reserve the EBDA area in the 'RAM size' value ...
     28 *
     29 * - The same quirk also avoids a problem with the AMD768MPX
     30 *   chipset: reserve a page before VGA to prevent PCI prefetch
     31 *   into it (errata #56). (Usually the page is reserved anyways,
     32 *   unless you have no PS/2 mouse plugged in.)
     33 *
     34 * - Plus paravirt systems don't have a reliable value in the
     35 *   'BIOS RAM size' pointer we can rely on, so we must quirk
     36 *   them too.
     37 *
     38 * Due to those various problems this function is deliberately
     39 * very conservative and tries to err on the side of reserving
     40 * too much, to not risk reserving too little.
     41 *
     42 * Losing a small amount of memory in the bottom megabyte is
     43 * rarely a problem, as long as we have enough memory to install
     44 * the SMP bootup trampoline which *must* be in this area.
     45 *
     46 * Using memory that is in use by the BIOS or by some DMA device
     47 * the BIOS didn't shut down *is* a big problem to the kernel,
     48 * obviously.
     49 */
     50
     51#define BIOS_RAM_SIZE_KB_PTR	0x413
     52
     53#define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
     54#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
     55
     56void __init reserve_bios_regions(void)
     57{
     58	unsigned int bios_start, ebda_start;
     59
     60	/*
     61	 * NOTE: In a paravirtual environment the BIOS reserved
     62	 * area is absent. We'll just have to assume that the
     63	 * paravirt case can handle memory setup correctly,
     64	 * without our help.
     65	 */
     66	if (!x86_platform.legacy.reserve_bios_regions)
     67		return;
     68
     69	/*
     70	 * BIOS RAM size is encoded in kilobytes, convert it
     71	 * to bytes to get a first guess at where the BIOS
     72	 * firmware area starts:
     73	 */
     74	bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
     75	bios_start <<= 10;
     76
     77	/*
     78	 * If bios_start is less than 128K, assume it is bogus
     79	 * and bump it up to 640K.  Similarly, if bios_start is above 640K,
     80	 * don't trust it.
     81	 */
     82	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
     83		bios_start = BIOS_START_MAX;
     84
     85	/* Get the start address of the EBDA page: */
     86	ebda_start = get_bios_ebda();
     87
     88	/*
     89	 * If the EBDA start address is sane and is below the BIOS region,
     90	 * then also reserve everything from the EBDA start address up to
     91	 * the BIOS region.
     92	 */
     93	if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
     94		bios_start = ebda_start;
     95
     96	/* Reserve all memory between bios_start and the 1MB mark: */
     97	memblock_reserve(bios_start, 0x100000 - bios_start);
     98}