AMD64: 64 bit kernel 32 bit userland - some pending questions

[Thomas Glanzmann <sithglan@stud.uni-erlangen.de>]

Hello everyone,
I would like to use an AMD64 Opteron System with a 64 bit Linux Kernel,
but a 32 bit userland (Debian Sarge). I have a few questions about this:

        - Is it possible to give the userland 3Gbyte virtual address
          space (default for 2.4 and 2.6). But give the Kernel a 64 bit
          virtual address space so that I get more than 1 Gbyte physical
          Memory into LOWMEM - say I want 8 Gbyte - without using HIGHMEM
          at all? If this scenario is possible I would get cheap memory
          access at the benefit of a well tested userland. I don't have
          applications that need more than 2 Gbyte virtual address
          space.

        - What is the easiest way to build a 64 bit kernel on a 32 bit
          Debian sarge. Are there crosscompiler packages available? Are
          there any guides on this?

        - If the above scenario works out like I imagine it, does this
          add some additional overhead I am not aware of when I switch
          for example from 32 bit userland to 64 bit kernel space which
          would override the performance gain I get from the huge LOWMEM
          virtuall address space?

        Thomas