Set environment variable `KVM` makes build fail

[Paul Menzel]

Dear Linux folks,


Playing with rcutorture 
(`tools/testing/selftests/rcutorture/bin/torture.sh`), a Linux kernel 
build failed, because the script sets and exports the environment 
variable `KVM`. I was able to reproduce that manually (on x86_64):

    $ make defconfig
    $ scripts/config -m KVM
    $ scripts/config -m KVM_INTEL
    $ scripts/config -d KVM_AMD
    $ scripts/config -d KVM_XEN
    $ scripts/config -d KVM_MMU_AUDIT
    $ export KVM=XXX
    $ make arch/x86/kvm/kvm.o
    […]
      LINK    /dev/shm/linux-kvm/tools/objtool/objtool
    make[2]: *** No rule to make target 'arch/x86/kvm/XXX/kvm_main.o', 
needed by 'arch/x86/kvm/kvm.o'.  Stop.
    make[1]: *** [scripts/Makefile.build:550: arch/x86/kvm] Error 2
    make: *** [Makefile:1831: arch/x86] Error 2

The directory `arch/x86/kvm/XXX` is created somehow.

Is that expected? Do you know what is going on? I wasn’t able to 
reproduce this with other variables like `SND_HDA` for example.


Kind regards,

Paul