Re: [uml-user] 64-bit UML with 32-bit root

[Michael R. <mc...@sa...>]

Mark Morgan Lloyd <mar...@te...> wrote:
    > The (unofficial?) FAQ at http://uml.devloop.org.uk/faq.html implies that
    > it is possible to build UML on a 64-bit system to run a 32-bit guest.

    > The best that I can manage on e.g. Debian "Jessie" x86-64 is to use
    > make ARCH=um SUBARCH=i386  which results in a 32-bit ELF to presumably
    > run a 32-bit guest. Prerequisites appear to be the multiarch-support
    > and gcc-multiarch packages.

    > Is it possible to build UML as a 64-bit binary, but to run a 32-bit
    > guest? What I'd like to be able to do is to put it on a system which has
    > no multiarch stuff, i.e. to completely sequester the 32-bit libraries
    > etc. within the guest filesystem.

You could try running that 64-bit kernel with a 32-bit binary.
Put a statically linked 32-bit busybox in the guest file system, or use a
32-bit Debian initrd as a test case..

I suspect that it won't work because the enclosing ("dom0") kernel will have
set the process to be a 64-bit process to run your kernel, and thus I think
the entire address space will be 64-bit.

    > [Background: I used UML fairly heavily in the 2.4 era, but I'm a
    > comparative newcomer to x86-64. I'm trying to avoid overuse of
    > multi-arch stuff.]

I am not sure why you are trying to avoid this, unless you are trying to run
the results on a system that doesn't have multi-arch.

(I'm fighting/putting-off replacing a Fedora10 build system with a Jessie
system, in great part because the appliance system needs a 64-bit kernel now,
but my build environment is 32-bit, and not-multiarch capable. Build-root
is the proper answer)

--
]               Never tell me the odds!                 | ipv6 mesh networks [
]   Michael Richardson, Sandelman Software Works        | network architect  [
]     mc...@sa...  http://www.sandelman.ca/        |   ruby on rails    [