[Bug 194923] kvm-intel.nested=1 does not enable nested KVM

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=194923

--- Comment #4 from R030t1@gmail.com ---
(In reply to Kashyap Chamarthy from comment #3)
> Based on your comment#2, there's not much useful information to diagnose
> this issue further.  You don't mention what kind of guest problem this is,
> or any other details.
> 

This problem was reported based on the behavior of a Windows guest, but I will
see what I can do in a Linux guest.

I run qemu as follows:

QEMU_AUDIO_DRV=pa sudo qemu-system-x86_64 \
        -machine accel=kvm \
        -cpu host,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time,+vmx,kvm=on \
        -smp 4 \
        -m 8G \
        -drive if=pflash,format=raw,readonly,file=OVMF_CODE-pure-efi.fd \
        -drive if=pflash,format=raw,file=OVMF_VARS.fd \
        -net tap,ifname=tap0,script=no,downscript=no -net nic,model=virtio \
        -drive file=/dev/sdc,format=raw,cache=none,if=virtio \
        -drive file=windows_10_pro.iso,media=cdrom \
        -drive file=../virtio-win-0.1.126.iso,media=cdrom \
        -usbdevice tablet \
        -vga qxl \
        -soundhw hda 

I found instructions
(https://ladipro.wordpress.com/2017/02/24/running-hyperv-in-kvm-guest/) which
gave me the options passed to the cpu flag. Using what the linked article gives
does not work; I have to pass kvm=on to have Hyper-V in the Windows guest
detect the ability to host VMs. It seems like passing kvm=on instructs qemu to
lie about functionality, but when using my most recent kernel which does not
support KVM nesting it properly reports that it is unable to pass
virtualization extensions to the guest.

When Hyper-V can detect VT-x on my processor it will let me create guests but
will be unable to run them:

    Failed to start the virtual machine 'WindowsServer' because one of the
Hyper-V components is not running.

It is my guess that the Windows service which acts like qemu is unable to start
due to being unable to use virtualization extensions due to some
incompatibility with real hardware.

> Please provide details about your guest OS, and related details --
> potentially `dmidecode`, `x86info -a`, `dmesg` from your physical host
> (level-0), and your level-1 guest (guest hypervisor).
> 

I'll do my best to attach what you've requested. I don't have very good guest
integration set up so getting some of the information out may take some time.

> Side note: A colleague who runs 4.9.13 Kernel reports that a Linux guest
> hypervisor was able to detect /dev/kvm just fine.

It may be hardware specific, but I am unsure why that would be. Considering the
type of relationship Windows has with the hypervisor and the virtual hardware I
am not sure it is right to blame it.

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