[PATCH] uml: allow finer tuning for host VMSPLIT setting

Now that various memory splits are enabled, add a config option allowing the
user to compile UML for its need - HOST_2G_2G allowed to choose either 3G/1G
or 2G/2G, and enabling it reduced the usable virtual memory.

Detecting this at run time should be implemented in the future, but we must
make the stop-gap measure work well enough (this is valid in _many_ cases).
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/Kconfig.i386

@@ -16,23 +16,42 @@ config SEMAPHORE_SLEEPERS
 	bool
 	default y
 
-config HOST_2G_2G
-	bool "2G/2G host address space split"
-	default n
-	help
-	This is needed when the host on which you run has a 2G/2G memory
-	split, instead of the customary 3G/1G.
-
-	Note that to enable such a host
-	configuration, which makes sense only in some cases, you need special
-	host patches.
-
-	So, if you do not know what to do here, say 'N'.
+choice
+	prompt "Host memory split"
+	default HOST_VMSPLIT_3G
+	---help---
+	   This is needed when the host kernel on which you run has a non-default
+	   (like 2G/2G) memory split, instead of the customary 3G/1G. If you did
+	   not recompile your own kernel but use the default distro's one, you can
+	   safely accept the "Default split" option.
+
+	   It can be enabled on recent (>=2.6.16-rc2) vanilla kernels via
+	   CONFIG_VM_SPLIT_*, or on previous kernels with special patches (-ck
+	   patchset by Con Kolivas, or other ones) - option names match closely the
+	   host CONFIG_VM_SPLIT_* ones.
+
+	   A lower setting (where 1G/3G is lowest and 3G/1G is higher) will
+	   tolerate even more "normal" host kernels, but an higher setting will be
+	   stricter.
+
+	   So, if you do not know what to do here, say 'Default split'.
+
+	config HOST_VMSPLIT_3G
+		bool "Default split (3G/1G user/kernel host split)"
+	config HOST_VMSPLIT_3G_OPT
+		bool "3G/1G user/kernel host split (for full 1G low memory)"
+	config HOST_VMSPLIT_2G
+		bool "2G/2G user/kernel host split"
+	config HOST_VMSPLIT_1G
+		bool "1G/3G user/kernel host split"
+endchoice
 
 config TOP_ADDR
 	hex
- 	default 0xc0000000 if !HOST_2G_2G
- 	default 0x80000000 if HOST_2G_2G
+	default 0xB0000000 if HOST_VMSPLIT_3G_OPT
+	default 0x78000000 if HOST_VMSPLIT_2G
+	default 0x40000000 if HOST_VMSPLIT_1G
+	default 0xC0000000
 
 config 3_LEVEL_PGTABLES
 	bool "Three-level pagetables (EXPERIMENTAL)"