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Subject[PATCH 14/14] uml: allow finer tuning for host VMSPLIT setting
From: Paolo 'Blaisorblade' Giarrusso <>

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 <>

arch/um/Kconfig.i386 | 49 ++++++++++++++++++++++++++++++++++---------------
1 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/arch/um/Kconfig.i386 b/arch/um/Kconfig.i386
index f6eb72d..f191a55 100644
--- a/arch/um/Kconfig.i386
+++ b/arch/um/Kconfig.i386
@@ -16,23 +16,42 @@ config SEMAPHORE_SLEEPERS
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'.
+ 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)"
+ 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"

config TOP_ADDR
- hex
- default 0xc0000000 if !HOST_2G_2G
- default 0x80000000 if HOST_2G_2G
+ hex
+ default 0xB0000000 if HOST_VMSPLIT_3G_OPT
+ default 0x78000000 if HOST_VMSPLIT_2G
+ default 0x40000000 if HOST_VMSPLIT_1G
+ default 0xC0000000

bool "Three-level pagetables (EXPERIMENTAL)"
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