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authorH. Peter Anvin <hpa@linux.intel.com>2013-02-27 12:46:40 -0800
committerH. Peter Anvin <hpa@linux.intel.com>2013-02-27 13:38:57 -0800
commit7c10093692ed2e6f318387d96b829320aa0ca64c (patch)
treef314c7fcd202c7c6e70fff730667f7f09c6f4a0e /arch/x86/kernel/head.c
parent058e7b5814534461b0e2468fce5a8f8d2f43c38f (diff)
download96b-common-7c10093692ed2e6f318387d96b829320aa0ca64c.tar.gz
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: Darren Hart <dvhart@linux.intel.com> Signed-off-by: H. 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
Diffstat (limited to 'arch/x86/kernel/head.c')
-rw-r--r--arch/x86/kernel/head.c53
1 files changed, 34 insertions, 19 deletions
diff --git a/arch/x86/kernel/head.c b/arch/x86/kernel/head.c
index 48d9d4ea1020..992f442ca155 100644
--- a/arch/x86/kernel/head.c
+++ b/arch/x86/kernel/head.c
@@ -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);