linux-kernel - Set environment variable `KVM` makes build fail

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Date:   Sun, 13 Feb 2022 07:03:10 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Zhouyi Zhou <zhouzhouyi@...il.com>
Subject: Set environment variable `KVM` makes build fail

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