Re: [PATCH 2/2] qemu_capabilities.c: drop 'kvm_pr' support for non-Power8 hosts

On 7/27/20 12:29 PM, Daniel P. Berrangé wrote: > On Fri, Jun 19, 2020 at 06:04:33PM -0300, Daniel Henrique Barboza wrote: > > PPC64 has two KVM modules: kvm_hv and kvm_pr. The official supported > > module was always kvm_hv, while kvm_pr was used for internal testing > > or for very niche cases in Power 8 hosts, always without official > > IBM or distro support. > > > > Problem is, QMP will report KVM supportfor PPC64 if any of these > > modules is loaded in the host, and kvm_pr is broken in everything > > but Power8 (and will remain broken, since kvm_pr is unmaintained). > > This can lead to situations like [1], where the tooling is misled to > > believe that the host has KVM capabilities when in reality it > > doesn't. > > > > The first reaction would be to simply forsake kvm_pr support entirely > > and move on, but there is no reason for now to be disruptive with any > > Power8 guests in the wild that are using kvm_pr (somehow). A more > > subtle approach is to not claim QEMU_CAPS_KVM support in all cases > > that we know it's completely broken, allowing Power8 users to take > > their shot using kvm_pr in their VMs. We can remove kvm_pr support > > completely when the module is removed from the kernel. > > I'm not sure libvirt should be forbidding this use of kvm_pr > on non-Power8. IIUC, it is the only impl that can actually be > used when in a Power9 LPARs. This patch is essentially saying > that TCG is better for Power LPARs than kvm_pr which I think > is not right. We didn't work in kvm_pr support for Power 9. All the effort was put into implementing nested virtualization with the kvm_hv module. The fact that kvm_pr module loads in Power 9 is kind of an "accident". As you can see in the bug linked, it's non-functional.