[src/trunk]: src/sys/arch/amd64/amd64 Explain why we should use kernel_map in...

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/060ea9bef1fc
branches:  trunk
changeset: 346227:060ea9bef1fc
user:      maxv <maxv%NetBSD.org@localhost>
date:      Sat Jul 02 07:22:09 2016 +0000

description:
Explain why we should use kernel_map instead of module_map, and why we
can't.

We should probably add some GCC flags in the modules makefiles to make
sure the relocations generated are not 32bit.

Related to PR/43438.

diffstat:

 sys/arch/amd64/amd64/machdep.c |  19 +++++++++++++++----
 1 files changed, 15 insertions(+), 4 deletions(-)

diffs (43 lines):

diff -r 3456f1fb8daf -r 060ea9bef1fc sys/arch/amd64/amd64/machdep.c
--- a/sys/arch/amd64/amd64/machdep.c    Fri Jul 01 23:29:17 2016 +0000
+++ b/sys/arch/amd64/amd64/machdep.c    Sat Jul 02 07:22:09 2016 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: machdep.c,v 1.219 2016/07/01 12:41:28 maxv Exp $       */
+/*     $NetBSD: machdep.c,v 1.220 2016/07/02 07:22:09 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 1.219 2016/07/01 12:41:28 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: machdep.c,v 1.220 2016/07/02 07:22:09 maxv Exp $");
 
 /* #define XENDEBUG_LOW  */
 
@@ -371,11 +371,22 @@
        minaddr = 0;
 
        /*
-        * Allocate a submap for physio
+        * Allocate a submap for physio.
         */
        phys_map = uvm_km_suballoc(kernel_map, &minaddr, &maxaddr,
-                                  VM_PHYS_SIZE, 0, false, NULL);
+           VM_PHYS_SIZE, 0, false, NULL);
 
+       /*
+        * 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.
+        *
+        * But because of the R_X86_64_32 relocations that are usually present
+        * in dynamic modules, the module map must be in low memory, and this
+        * wouldn't been guaranteed if we were using kernel_map.
+        */
        uvm_map_setup(&module_map_store, module_start, module_end, 0);
        module_map_store.pmap = pmap_kernel();
        module_map = &module_map_store;