KVM for 32bit arm had a get/set target mechanism to allow for
micro-architecture differences that are visible in system registers
to be described.

KVM's user-space can query the supported targets for a CPU, and
create vCPUs for that target. The target can override the handling
of system registers to provide different reset or RES0 behaviour.
On 32bit arm this was used to provide different ACTLR reset values
for A7 and A15.

On 64bit arm, the first few CPUs out of the gate used this mechanism,
before it was deemed redundant in commit bca556ac ("arm64/kvm:
Add generic v8 KVM target"). All future CPUs use the
KVM_ARM_TARGET_GENERIC_V8 target.

The 64bit target_table[] stuff exists to preserve the ABI to
user-space. As all targets registers genericv8_target_table, there
is no reason to look the target up.

Until we can merge genericv8_target_table with the main sys_regs
array, kvm_register_target_sys_reg_table() becomes
kvm_check_target_sys_reg_table(), which uses BUG_ON() in keeping
with the other callers in this file.
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200622113317.20477-2-james.morse@arm.com