x86/hyperv: Restrict get_vtl to only VTL platforms

When Linux runs in a non-default VTL (CONFIG_HYPERV_VTL_MODE=y),
get_vtl() must never fail as its return value is used in negotiations
with the host. In the more generic case, (CONFIG_HYPERV_VTL_MODE=n) the
VTL is always zero so there's no need to do the hypercall.

Make get_vtl() BUG() in case of failure and put the implementation under
"if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)" to avoid the call altogether in
the most generic use case.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/1695182675-13405-1-git-send-email-ssengar@linux.microsoft.com

Author: Saurabh Sengar

arch/x86/hyperv/hv_init.c

@@ -394,6 +394,7 @@ static void __init hv_get_partition_id(void)
 	local_irq_restore(flags);
 }
 
+#if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
 static u8 __init get_vtl(void)
 {
 	u64 control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_REGISTERS;
@@ -416,13 +417,16 @@ static u8 __init get_vtl(void)
 	if (hv_result_success(ret)) {
 		ret = output->as64.low & HV_X64_VTL_MASK;
 	} else {
-		pr_err("Failed to get VTL(%lld) and set VTL to zero by default.\n", ret);
-		ret = 0;
+		pr_err("Failed to get VTL(error: %lld) exiting...\n", ret);
+		BUG();
 	}
 
 	local_irq_restore(flags);
 	return ret;
 }
+#else
+static inline u8 get_vtl(void) { return 0; }
+#endif
 
 /*
  * This function is to be invoked early in the boot sequence after the
@@ -604,8 +608,7 @@ void __init hyperv_init(void)
 	hv_query_ext_cap(0);
 
 	/* Find the VTL */
-	if (!ms_hyperv.paravisor_present && hv_isolation_type_snp())
-		ms_hyperv.vtl = get_vtl();
+	ms_hyperv.vtl = get_vtl();
 
 	return;