Commit 7c100936 authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

x86: Make sure we can boot in the case the BDA contains pure garbage 



On non-BIOS platforms it is possible that the BIOS data area contains
garbage instead of being zeroed or something equivalent (firmware
people: we are talking of 1.5K here, so please do the sane thing.)

We need on the order of 20-30K of low memory in order to boot, which
may grow up to < 64K in the future.  We probably want to avoid the
lowest of the low memory.  At the same time, it seems extremely
unlikely that a legitimate EBDA would ever reach down to the 128K
(which would require it to be over half a megabyte in size.)  Thus,
pick 128K as the cutoff for "this is insane, ignore."  We may still
end up reserving a bunch of extra memory on the low megabyte, but that
is not really a major issue these days.  In the worst case we lose
512K of RAM.

This code really should be merged with trim_bios_range() in
arch/x86/kernel/setup.c, but that is a bigger patch for a later merge
window.

Reported-by: default avatarDarren Hart <dvhart@linux.intel.com>
Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/n/tip-oebml055yyfm8yxmria09rja@git.kernel.org
parent 058e7b58

arch/x86/kernel/head.c

+34 −19
Original line number Diff line number Diff line
@@ -5,8 +5,6 @@ 
#include <asm/setup.h>

#include <asm/bios_ebda.h>



#define BIOS_LOWMEM_KILOBYTES 0x413



/*

 * The BIOS places the EBDA/XBDA at the top of conventional

 * memory, and usually decreases the reported amount of
@@ -16,17 +14,30 @@ 
 * 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.

 *

 * This functions is deliberately very conservative.  Losing

 * memory in the bottom megabyte is rarely a problem, as long

 * as we have enough memory to install the trampoline.  Using

 * memory that is in use by the BIOS or by some DMA device

 * the BIOS didn't shut down *is* a big problem.

 */



#define BIOS_LOWMEM_KILOBYTES	0x413

#define LOWMEM_CAP		0x9f000U	/* Absolute maximum */

#define INSANE_CUTOFF		0x20000U	/* Less than this = insane */



void __init reserve_ebda_region(void)

{

	unsigned int lowmem, ebda_addr;



	/* To determine the position of the EBDA and the */

	/* end of conventional memory, we need to look at */

	/* the BIOS data area. In a paravirtual environment */

	/* that area is absent. We'll just have to assume */

	/* that the paravirt case can handle memory setup */

	/* correctly, without our help. */

	/*

	 * To determine the position of the EBDA and the

	 * end of conventional memory, we need to look at

	 * the BIOS data area. In a paravirtual environment

	 * that area is absent. We'll just have to assume

	 * that the paravirt case can handle memory setup

	 * correctly, without our help.

	 */

	if (paravirt_enabled())

		return;
@@ -37,19 +48,23 @@ void __init reserve_ebda_region(void) 
	/* start of EBDA area */

	ebda_addr = get_bios_ebda();



	/* Fixup: bios puts an EBDA in the top 64K segment */

	/* of conventional memory, but does not adjust lowmem. */

	if ((lowmem - ebda_addr) <= 0x10000)

		lowmem = ebda_addr;

	/*

	 * Note: some old Dells seem to need 4k EBDA without

	 * reporting so, so just consider the memory above 0x9f000

	 * to be off limits (bugzilla 2990).

	 */



	/* If the EBDA address is below 128K, assume it is bogus */

	if (ebda_addr < INSANE_CUTOFF)

		ebda_addr = LOWMEM_CAP;



	/* Fixup: bios does not report an EBDA at all. */

	/* Some old Dells seem to need 4k anyhow (bugzilla 2990) */

	if ((ebda_addr == 0) && (lowmem >= 0x9f000))

		lowmem = 0x9f000;

	/* If lowmem is less than 128K, assume it is bogus */

	if (lowmem < INSANE_CUTOFF)

		lowmem = LOWMEM_CAP;



	/* Paranoia: should never happen, but... */

	if ((lowmem == 0) || (lowmem >= 0x100000))

		lowmem = 0x9f000;

	/* Use the lower of the lowmem and EBDA markers as the cutoff */

	lowmem = min(lowmem, ebda_addr);

	lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */



	/* reserve all memory between lowmem and the 1MB mark */

	memblock_reserve(lowmem, 0x100000 - lowmem);

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