KVM: arm64: nv: Expose FEAT_ECBHB to NV-enabled VMs

KVM: arm64: nv: Expose FEAT_ECBHB to NV-enabled VMs

The exact wording of the restrictions on branch prediction due to
FEAT_ECBHB in DDI0487L.b is as follows:

  When FEAT_ECBHB is implemented, the branch history information created
  in a context before an exception to a higher Exception level using
  AArch64 cannot be used by code before that exception to exploitatively
  control the execution of any indirect branches in code in a different
  context after the exception.

While vEL2 and EL1 are multiplexed at EL1, they exist in different
hardware-described contexts as KVM uses different stage-2 MMUs to
represent the corresponding translation regimes. Additionally, exception
entries into vEL2 always imply a hardware exception entry into literal EL2
for the emulated regime change.

Given all of this, and the fact that FEAT_ECBHB places no limitation on
the EL of the protected context after the exception, we can claim
FEAT_ECBHB on supporting hardware.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>