hyperv: Detect Nested virtualization support for SVM

Previously, to detect nested virtualization enlightenment support,
we were using HV_X64_ENLIGHTENED_VMCS_RECOMMENDED feature bit of
HYPERV_CPUID_ENLIGHTMENT_INFO.EAX CPUID as docuemented in TLFS:
 "Bit 14: Recommend a nested hypervisor using the enlightened VMCS
  interface. Also indicates that additional nested enlightenments
  may be available (see leaf 0x4000000A)".

Enlightened VMCS, however, is an Intel only feature so the above
detection method doesn't work for AMD. So, use the
HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS.EAX CPUID information ("The
maximum input value for hypervisor CPUID information.") and this
works for both AMD and Intel.

Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Message-Id: <43b25ff21cd2d9a51582033c9bdd895afefac056.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/cpu/mshyperv.c

@@ -252,6 +252,7 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
 
 static void __init ms_hyperv_init_platform(void)
 {
+	int hv_max_functions_eax;
 	int hv_host_info_eax;
 	int hv_host_info_ebx;
 	int hv_host_info_ecx;
@@ -269,6 +270,8 @@ static void __init ms_hyperv_init_platform(void)
 	ms_hyperv.misc_features = cpuid_edx(HYPERV_CPUID_FEATURES);
 	ms_hyperv.hints    = cpuid_eax(HYPERV_CPUID_ENLIGHTMENT_INFO);
 
+	hv_max_functions_eax = cpuid_eax(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS);
+
 	pr_info("Hyper-V: privilege flags low 0x%x, high 0x%x, hints 0x%x, misc 0x%x\n",
 		ms_hyperv.features, ms_hyperv.priv_high, ms_hyperv.hints,
 		ms_hyperv.misc_features);
@@ -298,8 +301,7 @@ static void __init ms_hyperv_init_platform(void)
 	/*
 	 * Extract host information.
 	 */
-	if (cpuid_eax(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS) >=
-	    HYPERV_CPUID_VERSION) {
+	if (hv_max_functions_eax >= HYPERV_CPUID_VERSION) {
 		hv_host_info_eax = cpuid_eax(HYPERV_CPUID_VERSION);
 		hv_host_info_ebx = cpuid_ebx(HYPERV_CPUID_VERSION);
 		hv_host_info_ecx = cpuid_ecx(HYPERV_CPUID_VERSION);
@@ -325,9 +327,11 @@ static void __init ms_hyperv_init_platform(void)
 			ms_hyperv.isolation_config_a, ms_hyperv.isolation_config_b);
 	}
 
-	if (ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
+	if (hv_max_functions_eax >= HYPERV_CPUID_NESTED_FEATURES) {
 		ms_hyperv.nested_features =
 			cpuid_eax(HYPERV_CPUID_NESTED_FEATURES);
+		pr_info("Hyper-V: Nested features: 0x%x\n",
+			ms_hyperv.nested_features);
 	}
 
 	/*