KVM: arm64: limit PMU version to PMUv3 for ARMv8.1

We currently expose the PMU version of the host to the guest via
emulation of the DFR0_EL1 and AA64DFR0_EL1 debug feature registers.
However many of the features offered beyond PMUv3 for 8.1 are not
supported in KVM. Examples of this include support for the PMMIR
registers (added in PMUv3 for ARMv8.4) and 64-bit event counters
added in (PMUv3 for ARMv8.5).

Let's trap the Debug Feature Registers in order to limit
PMUVer/PerfMon in the Debug Feature Registers to PMUv3 for ARMv8.1
to avoid unexpected behaviour.

Both ID_AA64DFR0.PMUVer and ID_DFR0.PerfMon follow the "Alternative ID
scheme used for the Performance Monitors Extension version" where 0xF
means an IMPLEMENTATION DEFINED PMU is implemented, and values 0x0-0xE
are treated as with an unsigned field (with 0x0 meaning no PMU is
present). As we don't expect to expose an IMPLEMENTATION DEFINED PMU,
and our cap is below 0xF, we can treat these fields as unsigned when
applying the cap.

Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
[Mark: make field names consistent, use perfmon cap]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>