KVM: arm64: Add some more comments in kvm_hyp_handle_fpsimd()

The handling for FPSIMD/SVE traps is multi stage and involves some trap
manipulation which isn't quite so immediately obvious as might be desired
so add a few more comments.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220124155720.3943374-3-broonie@kernel.org

arch/arm64/kvm/hyp/include/hyp/switch.h

@@ -173,6 +173,8 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
 		return false;
 
 	/* Valid trap.  Switch the context: */
+
+	/* First disable enough traps to allow us to update the registers */
 	if (has_vhe()) {
 		reg = CPACR_EL1_FPEN;
 		if (sve_guest)
@@ -188,11 +190,13 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
 	}
 	isb();
 
+	/* Write out the host state if it's in the registers */
 	if (vcpu->arch.flags & KVM_ARM64_FP_HOST) {
 		__fpsimd_save_state(vcpu->arch.host_fpsimd_state);
 		vcpu->arch.flags &= ~KVM_ARM64_FP_HOST;
 	}
 
+	/* Restore the guest state */
 	if (sve_guest)
 		__hyp_sve_restore_guest(vcpu);
 	else