Modifying qemu without screwing up my current install

Question

I currently have kvm installed on my Debian laptop:

kvm --version
QEMU emulator version 2.1.2 (Debian 1:2.1+dfsg-12+deb8u5a),

I want to modify it to supported nested virtualization (ESXi VMs).

This little tutorial seems to do exactly what I want, but I'm confused about this part:

Patch and compile QEMU

cd /opt
sudo git clone https://github.com/qemu/qemu.git
cd qemu
sudo curl "https://gist.githubusercontent.com/jcpowermac/3d9c732be08404302083/raw/ba97ceceefb2ffb085fa8da0f5f5a6142127454e/qemu.patch" | sudo patch -p1
sudo ./configure --enable-kvm --target-list=x86_64-linux-user,x86_64-softmmu
sudo make -j8
sudo wget "https://gist.githubusercontent.com/jcpowermac/36bfa62cd60781264b3f/raw/f26aa286d5ab85f17555141e04ab549e10727475/qemu-kvm"

He says after doing this:

This will leave our original QEMU install untouched, which is probably a good thing. Next up we need to define a virtual machine.

But that leaves me wondering how do I use this modified version instead of my current version? Building the modified qemu in this way really won't overwrite my current version? When I run virsh which version will it be pointing to?

Answer 1

Simply reach it through it's fully qualified path, it will be installed in /opt. As mentionned in the tutorial, the domain XML from libvirt includes a section which points that Virtual Machine to the correct QEMU program to use.

I fail to see the point of the aforementionned patch though, as it seems to simply override the vmport setting that can be set in libvirt XML to begin with... Last I heard, the only thing required to do nesting with KVM is to enable it in the module through modprobe.d for example. You might want to try without the patch first, or find a more up-to-date source.