[PATCH v5 10/27] KVM: arm64: Hide IMPLEMENTATION DEFINED PMU support for the guest

[Oliver Upton]

Hi Reiji,

On Sun, Feb 13, 2022 at 10:57:29PM -0800, Reiji Watanabe wrote:
> When ID_AA64DFR0_EL1.PMUVER or ID_DFR0_EL1.PERFMON is 0xf, which
> means IMPLEMENTATION DEFINED PMU supported, KVM unconditionally
> expose the value for the guest as it is.  Since KVM doesn't support
> IMPLEMENTATION DEFINED PMU for the guest, in that case KVM should
> expose 0x0 (PMU is not implemented) instead.
> 
> Change cpuid_feature_cap_perfmon_field() to update the field value
> to 0x0 when it is 0xf.

Definitely agree with the change in this patch. Do we need to tolerate
writes of 0xf for ABI compatibility (even if it is nonsensical)?
Otherwise a guest with IMP_DEF PMU cannot be migrated to a newer kernel.

--
Thanks,
Oliver