kvm-userspace requires kvm capable kernel headers in default search

[Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>]

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Hi everyone,
while running a test when updating kvm-userspace for powerpc I found 
that the current kvm userspace requires kvm kernel headers in the 
default search path of the used compilers.
I used to update and build in the same kvm-userspace directory for a 
while and this one had an old stale kernel/include directory which 
fulfilled all the requirements.
While testing my current patch series on a new git clone of 
kvm-userspace I wondered why this doesn't work anymore (it worked in my 
old directory which also had the updated current source).

I switched to x86 to verify that issue there and I found that eventually 
the issue is in the kvm detection "KVM Probe" part of the qemu 
configuration in kvm-userspace. It failed not finding kvm headers.
   gcc -m32 -o /tmp/qemu-conf--21885-  
-I/home/paelzer/Desktop/kvm-userspace/kernel/include 
/tmp/qemu-conf--21885-.c
   /tmp/qemu-conf--21885-.c:1:23: error: linux/kvm.h: No such file or 
directory
   /tmp/qemu-conf--21885-.c:3:2: error: #error Invalid KVM version

Looking at the include paths it is worth to note that a current git 
clone of kvm-userspace has no kernel/include directory anymore. A few 
questions later to some other kvm developers I found that there headers 
can be found, but in the default search path of the compiler e.g. 
/usr/include. In my environment with an older gcc cross compiler for 
powerpc and no up to date linux headers installed for x86 both 
architectures failed.

The Solution to that can be done in several ways:
a) we decide that kvm-userspace needs up-to-date kernel headers 
installed. And modify the KVM Probe at least to tell the user about this 
possible reason when failing instead of silently switching to "KVM 
support no".
b) if the user already provide a --kerneldir option to specify where the 
right includes can be found we should give those configure checks a 
chance to really reach that. Atm it adds the kernel/include path of the 
kvm-userspace tree which doesn't exist anymore (except as stale old dir 
in a lot of source trees out there).
c) I overlooked something and there is an even better or trivial 
approach :-)

Since this could be solved several very different ways I think its worth 
a discussion.
As I prefer b) I attached a simple patch example how this could look 
like :-).

-- 

Grüsse / regards, 
Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization


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diff --git a/configure b/configure
index 63f956c..f772bae 100755
--- a/configure
+++ b/configure
@@ -3,6 +3,7 @@
 prefix=/usr/local
 kernelsourcedir=
 kerneldir=/lib/modules/$(uname -r)/build
+qemu_kerneldir=
 cc=gcc
 ld=ld
 objcopy=objcopy
@@ -56,6 +57,7 @@ while [[ "$1" = -* ]]; do
 	    ;;
 	--kerneldir)
 	    kerneldir="$arg"
+	    qemu_kerneldir="$arg"
 	    ;;
 	--with-patched-kernel)
 	    want_module=
@@ -84,9 +86,12 @@ while [[ "$1" = -* ]]; do
     esac
 done
 
-
-#set kenel directory
+#set libkvm kernel directory
 libkvm_kerneldir=$(readlink -f kernel)
+# use libkvm_kerneldir for qemu if no kerneldir option was set
+if test "$qemu_kerneldir" = "" ; then
+    qemu_kerneldir=$libkvm_kerneldir
+fi
 
 case $arch in
     i?86*|x86_64*)
@@ -123,7 +128,7 @@ fi
     --disable-gcc-check \
     --extra-cflags="-I $PWD/../libkvm $qemu_cflags" \
     --extra-ldflags="-L $PWD/../libkvm $qemu_ldflags" \
-    --kerneldir="$libkvm_kerneldir" \
+    --kerneldir="$qemu_kerneldir" \
     --prefix="$prefix" \
     ${cross_prefix:+"--cross-prefix=$cross_prefix"} \
     ${cross_prefix:+"--cpu=$arch"} "${qemu_opts[@]}"