Hi all, the actual capabilities of QEMU are depending on the host. This includes dependencies like which kernel modules are loaded or which kernel parameters are used (e.g. kvm.nested). Therefore, after a restart we cannot be sure that the QEMU capabilities remain the same. How can we solve this problem?

I have come up with two ways: - reprobe the capabilities with every host reboot

- check for every possible change in virQEMUCapsIsValid... (this is already done for KVM). In my opinion this is not the way to go.

On Fri, Oct 20, 2017 at 01:10 PM +0200, Pavel Hrdina <phrdina(a)redhat.com&gt; wrote: > On Fri, Oct 20, 2017 at 12:59:51PM +0200, Marc Hartmayer wrote: >> Hi all, >> >> the actual capabilities of QEMU are depending on the host. This >> includes dependencies like which kernel modules are loaded or which >> kernel parameters are used (e.g. kvm.nested). Therefore, after a >> restart we cannot be sure that the QEMU capabilities remain the same. >> >> How can we solve this problem? > > Hi, > > thanks for bringing up this issue, we kind of already know about it > but it's good idea to discuss it publicly. > >> >> I have come up with two ways: >> - reprobe the capabilities with every host reboot > > This is the solution that was agreed on but nobody was motivated enough > to write the code :). First - thank you for the quick answer :) Would it be okay if the solution depends on systemd?

On Fri, Oct 20, 2017 at 01:10:14PM +0200, Pavel Hrdina wrote: > On Fri, Oct 20, 2017 at 12:59:51PM +0200, Marc Hartmayer wrote: > > Hi all, > > > > the actual capabilities of QEMU are depending on the host. This > > includes dependencies like which kernel modules are loaded or which > > kernel parameters are used (e.g. kvm.nested). Therefore, after a > > restart we cannot be sure that the QEMU capabilities remain the same. > > > > How can we solve this problem? > > Hi, > > thanks for bringing up this issue, we kind of already know about it > but it's good idea to discuss it publicly. > > > > > I have come up with two ways: > > - reprobe the capabilities with every host reboot > > This is the solution that was agreed on but nobody was motivated enough > to write the code :). Reprobing QEMU is quite heavy. I wonder if we should change slightly. Make our capabilities cache *only* contain stuff reported by the QEMU binary. For other stuff we detect from the host, never cache it.