Module Name: src
Committed By: maxv
Date: Tue Nov 15 15:26:59 UTC 2016
Modified Files:
src/sys/arch/amd64/amd64: machdep.c
Log Message:
I actually came across the solution to this issue in the Intel SDM for a
totally unrelated reason a few weeks ago. The reason we need a particular
module_map on amd64 is because gcc makes us use RIP-relative addressing.
The offset field of the associated opcodes is a 32bit signed displacement,
which means we can access only up to 2GB around the current instruction.
And given that kernel_map is too far away from the kernel .text, it is not
RIP-addressable. Hence the module_map embedded into the bootstrap memory,
which is right above the kernel image.
To generate a diff of this commit:
cvs rdiff -u -r1.231 -r1.232 src/sys/arch/amd64/amd64/machdep.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/amd64/amd64/machdep.c
diff -u src/sys/arch/amd64/amd64/machdep.c:1.231 src/sys/arch/amd64/amd64/machdep.c:1.232
--- src/sys/arch/amd64/amd64/machdep.c:1.231 Tue Nov 15 15:00:56 2016
+++ src/sys/arch/amd64/amd64/machdep.c Tue Nov 15 15:26:59 2016
@@ -1,4 +1,4 @@
-/* $NetBSD: machdep.c,v 1.231 2016/11/15 15:00:56 maxv Exp $ */
+/* $NetBSD: machdep.c,v 1.232 2016/11/15 15:26:59 maxv Exp $ */
/*-
* Copyright (c) 1996, 1997, 1998, 2000, 2006, 2007, 2008, 2011
@@ -111,7 +111,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: machdep.c,v 1.231 2016/11/15 15:00:56 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: machdep.c,v 1.232 2016/11/15 15:26:59 maxv Exp $");
/* #define XENDEBUG_LOW */
@@ -377,12 +377,14 @@ cpu_startup(void)
/*
* Create the module map.
*
- * XXX: the module map is taken as what is left of the bootstrap memory
- * created in locore.S, which is not big enough if we want to load many
- * modules dynamically. We really should be using kernel_map instead.
+ * The kernel uses RIP-relative addressing with a maximum offset of
+ * 2GB. The problem is, kernel_map is too far away in memory from
+ * the kernel .text. So we cannot use it, and have to create a
+ * special module_map.
*
- * But the modules must be located above the kernel image, and that
- * wouldn't be guaranteed if we were using kernel_map.
+ * The module map is taken as what is left of the bootstrap memory
+ * created in locore.S. This memory is right above the kernel
+ * image, so this is the best place to put our modules.
*/
uvm_map_setup(&module_map_store, module_start, module_end, 0);
module_map_store.pmap = pmap_kernel();
