[PATCH v2 11/14] KVM: arm64: PMU: Allow ID_AA64DFR0_EL1.PMUver to be set from userspace

[Reiji Watanabe]

Hi Marc,

> > BTW, if we have no intention of supporting a mix of vCPUs with and
> > without PMU, I think it would be nice if we have a clear comment on
> > that in the code.  Or I'm hoping to disallow it if possible though.
>
> I'm not sure we're in a position to do this right now. The current API
> has always (for good or bad reasons) been per-vcpu as it is tied to
> the vcpu initialisation.

Thank you for your comments!
Then, when a guest that has a mix of vCPUs with and without PMU,
userspace can set kvm->arch.dfr0_pmuver to zero or IMPDEF, and the
PMUVER for vCPUs with PMU will become 0 or IMPDEF as I mentioned.
For instance, on the host whose PMUVER==1, if vCPU#0 has no PMU(PMUVER==0),
vCPU#1 has PMU(PMUVER==1), if the guest is migrated to another host with
same CPU features (PMUVER==1), if SET_ONE_REG of ID_AA64DFR0_EL1 for vCPU#0
is done after for vCPU#1, kvm->arch.dfr0_pmuver will be set to 0, and
the guest will see PMUVER==0 even for vCPU1.

Should we be concerned about this case?

Thank you,
Reiji