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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)"
    + 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"

    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

    config 3_LEVEL_PGTABLES
    bool "Three-level pagetables (EXPERIMENTAL)"
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      Last update: 2006-10-05 23:47    [W:0.023 / U:44.680 seconds]
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