Public bug reported:
[impact]
libvirt caches the 'nested vmx' capability of the host and does not
update that even if the host's capability to handle nested vmx changes.
Having this domcapability missing means no guests are able to start any
nested, kvm-accelerated, guests. Additionally, since openstack live
migration requires matching cpu features, this makes migrating guests
that do have vmx enabled impossible to hosts where libvirt thinks nested
vmx is disabled.
Once the kernel module (kvm_intel) is reloaded with 'nested' enabled,
libvirt does not update its domcapabilities cache, even over a libvirtd
restart, or even over an entire system reboot. Only certain conditions
cause libvirt to update its capabilities cache (possibly libvirt
upgrade, or qemu upgrade, or kernel upgrade...I haven't verified any of
those yet)
libvirt creates caches for its domcapabilities at
/var/cache/libvirt/qemu/capabilities/.
removing the cache xml files there and restarting libvirtd will cause the
caches to be recreated with the correct current values.
[test case]
check the kvm_intel module nested parameter:
$ cat /sys/module/kvm_intel/parameters/nested
Y
it can be Y or N. make sure libvirt agrees with the current setting:
$ virsh domcapabilities | grep vmx
<feature policy='require' name='vmx'/>
if 'nested' is Y, domcapabilities should include a vmx feature line; if
'nested' is N, it should have no output (i.e. vmx not supported in
guests).
Then, change the kernel nested setting, and re-check domcapabilities.
Restarting libvirtd doesn't update the cache, and even rebooting the
entire system doesn't update the cache.
$ virsh domcapabilities | grep vmx
$ cat /sys/module/kvm_intel/parameters/nested
N
$ sudo rmmod kvm_intel
$ sudo modprobe kvm_intel nested=1
$ cat /sys/module/kvm_intel/parameters/nested
Y
$ virsh domcapabilities | grep vmx
$ sudo systemctl restart libvirtd
$ virsh domcapabilities | grep vmx
$
[regression potential]
TBD, but this should only require better invalidating of the caps cache,
which seems very low for any regression potential. More overhead to
regenerate the cache more often maybe.
[other info]
** Affects: libvirt (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1830268
Title:
libvirt caches nested vmx capability (in domcapabilities)
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