[RFC] use realmode code to reserve end-of-conventional-memory to 1MB

[Alexander van Heukelum <heukelum@mailshack.com>]

Instead of using early reservations inside the kernel code,
we could use the realmode code to modify the e820 memmap.
This patch shows what that would look like. I have not looked
at the case where the BIOS does not provide an e820 memmap
yet. Probably a full solution would need to create a fake
e820 memmap in that case.

Comments?

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>

diff --git a/arch/x86/boot/memory.c b/arch/x86/boot/memory.c
index e77d89f..522920a 100644
--- a/arch/x86/boot/memory.c
+++ b/arch/x86/boot/memory.c
@@ -23,6 +23,7 @@ static int detect_memory_e820(void)
 	int count = 0;
 	u32 next = 0;
 	u32 size, id;
+	u32 lowmem, ebda_addr;
 	u8 err;
 	struct e820entry *desc = boot_params.e820_map;
 
@@ -50,13 +51,51 @@ static int detect_memory_e820(void)
 		   just return failure. */
 		if (id != SMAP) {
 			count = 0;
-			break;
+			goto out;
 		}
 
 		count++;
 		desc++;
-	} while (next && count < E820MAX);
-
+	} while (next && count < (E820MAX - 1));
+
+	/* Some BIOSes do not reserve the EBDA/XBDA area correctly in.
+	   The e820-map. Find out where the EBDA resides by looking at
+	   the BIOS data area and reserve the EBDA and the following
+	   legacy adapter area explicitly. */
+#define BIOS_EBDA_SEGMENT 0x40E
+#define BIOS_LOWMEM_KILOBYTES 0x413
+
+	/* end of low (conventional) memory */
+	set_fs(0);
+	lowmem = rdfs16(BIOS_LOWMEM_KILOBYTES);
+	lowmem <<= 10;
+
+	/* start of EBDA area */
+	ebda_addr = rdfs16(BIOS_EBDA_SEGMENT);
+	ebda_addr <<= 4;
+
+	/* 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;
+
+	/* 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;
+
+	/* Paranoia: should never happen, but... */
+	if (lowmem >= 0x100000)
+		lowmem = 0xa0000;
+
+	/* reserve all memory between lowmem and the 1MB mark */
+	desc->addr = lowmem;
+	desc->size = 0x100000 - lowmem;
+	desc->type = E820_RESERVED;
+	count++;
+	desc++;
+
+out:
 	return boot_params.e820_entries = count;
 }