Comment 6 for bug 1778854

------- Comment From <email address hidden> 2018-12-27 14:20 EDT-------
Hello,

I could reproduce this on Power9, in the following kernels:
4.15.0-20-generic
4.18.0-10-generic

I compiled kvm_pr for 4.18, and debbuged it until I found this:

arch/powerpc/kvm/book3s_pr.c:2038
/*
* PR KVM can work on POWER9 inside a guest partition
* running in HPT mode. It can't work if we are using
* radix translation (because radix provides no way for
* a process to have unique translations in quadrant 3).
*/
if (cpu_has_feature(CPU_FTR_ARCH_300) && radix_enabled())
return -EIO;
return 0;

Here, cpu_has_feature(CPU_FTR_ARCH_300) returns:
false for Power8,
true for Power9.

And radix_enabled() returns:
false if bootargs contains "disable_radix",
true otherwise.

##

Meaning that, if one wants to load the kvm_pr module on a guest with Power9, the guest kernel must be booted with "disable_radix", as it looks like radix is the default choice.

paelzer, please verify if your tests run with this parameter.

##

BTW, it only works on vanilla kernel v4.18+. Before that, kvm_pr was just disabled for Power9. I am not sure about Ubuntu kernel including this patch on earlier kernels.