Xen nested inside dom0 or PV not working

[techman <techman@xxxxxxxxx>]

I try to get a nested VM.

Specifically I wanted to have a QEMU guest inside a Xen dom0 or a Xen PV 
on Debian Bookworm (11).

xen_version            : 4.14.5


Of course I switched on vt-x in the BIOS and everything related.

When starting the kernel without the hypervisor I have normal 
Virtualbox, QEMU and KVM working.
When starting with the Xen hypervisor no more virtualization works.


Booted dom0 with hypervisor:
$ cat /proc/cpuinfo | grep ^flags | tail -n1
flags        : fpu de tsc msr pae mce cx8 apic sep mca cmov pat clflush 
acpi mmx fxsr sse sse2 ss ht syscall nx rdtscp lm constant_tsc rep_good 
nopl nonstop_tsc cpuid tsc_known_freq pni pclmulqdq monitor est ssse3 
fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand hypervisor 
lahf_lm abm cpuid_fault ssbd ibrs ibpb stibp fsgsbase bmi1 avx2 bmi2 
erms xsaveopt md_clear
$ cat /proc/cpuinfo| grep vmx | tail -n1

Booted without hypervisor:
$ cat /proc/cpuinfo | grep ^flags | tail -n1
flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca 
cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall 
nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl 
xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor 
ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 
x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm 
abm cpuid_fault invpcid_single pti intel_ppin ssbd ibrs ibpb stibp 
tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 
avx2 smep bmi2 erms invpcid cqm xsaveopt cqm_llc cqm_occup_llc dtherm 
ida arat pln pts md_clear flush_l1d
$ cat /proc/cpuinfo | grep vmx | tail -n1
vmx flags    : vnmi preemption_timer posted_intr invvpid ept_x_only 
ept_ad ept_1gb flexpriority apicv tsc_offset vtpr mtf vapic ept vpid 
unrestricted_guest vapic_reg vid ple

As the manual says I put

hap=1
nestedhvm=1

in the vm.cfg for the PV with which I create the VM. It has no effect.


lshw says this about my mainboard and CPU when booting without the 
hypervisor:
  *-core
       description: Motherboard
       product: X99A SLI PLUS(MS-7885)
       vendor: MSI
     *-cpu
          description: CPU
          product: Intel(R) Core(TM) i7-5960X CPU @ 3.00GHz
          vendor: Intel Corp.
          physical id: 69
          bus info: cpu@0
          version: Intel(R) Core(TM) i7-5960X CPU @ 3.00GHz
          slot: SOCKET 0
          size: 1539MHz
          capacity: 4GHz
          width: 64 bits
          clock: 100MHz
          capabilities: lm fpu fpu_exception wp vme de pse tsc msr pae 
mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr 
sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp x86-64 constant_tsc 
arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid 
aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg 
fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt 
tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm cpuid_fault 
invpcid_single pti intel_ppin ssbd ibrs ibpb stibp tpr_shadow vnmi 
flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 
erms invpcid cqm xsaveopt cqm_llc cqm_occup_llc dtherm ida arat pln pts 
md_clear flush_l1d cpufreq
          configuration: cores=8 enabledcores=8 threads=16



So how can I proceed from here?

What can I try next?

How to diagnose the problem?


Please help!