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

[Daniel P. Berrangé]

On Mon, Jul 27, 2020 at 01:44:07PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> 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.

If kvm_pr is known to be broken in Power 9 LPARs, then that is an
upstream kernel bug for allowing it to be loaded under Power 9 at
all. Can we fix the kernel to prevent it loading in cases where it
is known to be bad.

Regards,
Daniel
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