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);
+}