On Fri, Jul 07, 2023 at 09:07:54AM +0000, Saurabh Singh Sengar wrote:

[...]

quoted

+static u8 __init get_vtl(void)
+{
+	u64 control = HV_HYPERCALL_REP_COMP_1 |
HVCALL_GET_VP_REGISTERS;
+	struct hv_get_vp_registers_input *input;
+	struct hv_get_vp_registers_output *output;
+	u64 vtl = 0;
+	u64 ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
+	output = (struct hv_get_vp_registers_output *)input;
+	if (!input) {
+		local_irq_restore(flags);
+		goto done;
+	}
+
+	memset(input, 0, struct_size(input, element, 1));
+	input->header.partitionid = HV_PARTITION_ID_SELF;
+	input->header.vpindex = HV_VP_INDEX_SELF;
+	input->header.inputvtl = 0;
+	input->element[0].name0 = HV_X64_REGISTER_VSM_VP_STATUS;
+
+	ret = hv_do_hypercall(control, input, output);
+	if (hv_result_success(ret))
+		vtl = output->as64.low & HV_X64_VTL_MASK;
+	else
+		pr_err("Hyper-V: failed to get VTL! %lld", ret);

In case of error this function will return vtl=0, which can be the valid value of vtl.
I suggest we initialize vtl with -1 so and then check for its return.

This could be a good utility function which can be used for any Hyper-V VTL system, so think
of making it global ?

Tianyu -- your thought on this?