It's QEMU and the kernel that do the magic - libvirt is actually kinda annoying and ill suited for these things, you have to edit lots of unnecessary XML just to change launch options for QEMU.

That sounds very interesting. Does that mean I should be able to load a VM containing my windows partition fron linux, even without a dedicated GPU? Do you have a pointer to a tutorial explaining the setup ?

Yes, I can confirm that this works. I have one windows installation and can either directly boot windows on the host or start it within Linux with VFIO.

Do you know where this is documented? This would be very useful to me.

I wrote a `viewsetup` tool for setting up a /dev/mapper device (and VMDK file pointing to it) that I use for exposing a set of native Windows partitions while masking off the active Linux partitions. It's enough to allow booting Windows natively or in a VirtualBox VM using the same partition. But the steps to create backing files, run dm-setup to create the /dev/mapper device, and create the VMDK are all separate, so you ought to be able to use this with any virtualization tool.

I sort of have it documented at https://github.com/hotsphink/sfink-tools/blob/master/doc/Vir... though that's really a set of instructions intended for a new PC that a bunch of us at my employer recently received. You just want the `viewsetup` stuff. You probably want to leave off the `--auto` flag, so that it'll prompt you per-partition.

The tool goes to some effort to only expose the specific partitions needed. It also makes the virtual disk exactly match the native disk, copying small partitions (and gaps between them) to files and exposing those over loopback interfaces, so that Windows can go crazy and write over everything and it won't break your Linux setup. (The Linux partitions themselves are exposed as loopback devices to empty sparse files of the right size.)

Get the single script file at https://hg.sr.ht/~sfink/sfink-tools/raw/bin/viewsetup?rev=ti... or check out the full repo at either https://hg.sr.ht/~sfink/sfink-tools/ (mercurial) or https://github.com/hotsphink/sfink-tools (git). I keep both up to date.

Thanks! I will test this out later.

> That sounds very interesting. Does that mean I should be able to load a VM containing my windows partition fron linux, even without a dedicated GPU?

That works fine.

I do that at work to avoid nuking/tampering with the Windows installation provided to me by IT, while running Linux as my main OS from a second volume.

How is the IO (particularly random IO) performance in Windows? In my experience that's been where VMs still have a long ways to go. Even kvm2 Linux on Linux VMs have severe IO performance deficits compared to native.