[PATCH] Fix value of mbi->mem_lower for multiboot-x86

[Peter Chubb]

In the multiboot header, there is a field, `mem_lower' that is meant to
contain the size of memory starting at zero and ending below 640k.
If your kernel is compiled with CONFIG_X86_RESERVE_LOW non zero
(the usual case), then a hole is inserted into kernel's physical
memory map at zero, so the test to find the size of this region in
kexec/arch/i386/kexec-multiboot-x86.c never succeeds, so the value is
always zero.

On a PC99 architecture, there is always memory at physycal address zero;
assume that a region that starts below 64k actually starts at zero,
and use it for the mem_lower variable.

Signed-off-by: Peter Chubb <peter.chubb at nicta.com.au>

---
 kexec/arch/i386/kexec-multiboot-x86.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

Index: kexec-tools-2.0.4/kexec/arch/i386/kexec-multiboot-x86.c
===================================================================
--- kexec-tools-2.0.4.orig/kexec/arch/i386/kexec-multiboot-x86.c	2013-03-14 18:45:16.000000000 +1000
+++ kexec-tools-2.0.4/kexec/arch/i386/kexec-multiboot-x86.c	2014-01-15 10:21:02.138172304 +1000
@@ -261,10 +261,18 @@ int multiboot_x86_load(int argc, char **
 		mmap[i].length_high    = length >> 32;
 		if (range[i].type == RANGE_RAM) {
 			mmap[i].Type = 1; /* RAM */
-			/* Is this the "low" memory? */
-			if ((range[i].start == 0)
-			    && (range[i].end > mem_lower))
+			/*
+                         * Is this the "low" memory?  Can't just test
+                         * against zero, because Linux protects (and
+                         * hides) the first few pages of physical
+                         * memory.
+                         */
+
+			if ((range[i].start <= 64*1024)
+			    && (range[i].end > mem_lower)) {
+                                range[i].start = 0;
 				mem_lower = range[i].end;
+                        }
 			/* Is this the "high" memory? */
 			if ((range[i].start <= 0x100000)
 			    && (range[i].end > mem_upper + 0x100000))

--
Dr Peter Chubb				        peter.chubb AT nicta.com.au
http://www.ssrg.nicta.com.au          Software Systems Research Group/NICTA