But who will check if the target supports PSCI 1.2 (or later)? All my FDT-based boards hang inside the arm_smccc_1_2_hvc() function (none of them support PSCI 1.2).

Huh, this function is obviously wrong and incomplete . We don't always run on a hypervisor and don't need SMCCC 1.2. There is no check for return values in hv_get_vpreg_128() .
I'm afraid this code was newer tested on a system without hyper-v

Thanks for pointing, I have just found this problem. Will add a check with AcpiGbl_FADT HypervisorId to check if we are on Hyper-V or not ,
based on that will return.
Proposed change:
bool
hyperv_identify_features(void)
{
struct hv_get_vp_registers_output result;

        if (AcpiGbl_FADT.HypervisorId == "MsHyperv")
	   vm_guest = VM_GUEST_HV;
        else
            return (False);

hv_get_vpreg_128(CPUID_LEAF_HV_FEATURES, &result);
hyperv_features = result.as32.a;
hv_get_vpreg_128(CPUID_LEAF_HV_IDENTITY, &result);
hyperv_ver_major = result.as32.b >> 16;
hv_get_vpreg_128(CPUID_LEAF_HV_RECOMMENDS, &result);
hyperv_recommends = result.as32.a;
return (true);
}
will put this change, which should solve the problem.

WIll put the new patch for review by tomorrow.