Hello Tom,

On Mon, Aug 09, 2021 at 08:41:27AM -0500, Tom Lendacky wrote:

On 8/2/21 7:31 AM, Ashish Kalra wrote:

+
+ Signature[12] = '\0';
+ for (mKvmLeaf = 0x40000000; mKvmLeaf < 0x40010000; mKvmLeaf += 0x100) {

I still really don't understand the need for the CPUID loop. KVM only ever
programs CPUID function 0x40000000, right?

Yes KVM only programs CPUID function 0x40000000, as do other hypervisors
like Hyper-V. Also mentioned that leaf 0x40000000 is the Hypervisor
CPUID leaf range and vendor ID signature in MSFT Hypervisor Interface
document.

But looking at linux kernel code for the same functionality :

static inline uint32_t hypervisor_cpuid_base(const char *sig, uint32_t leaves)
{
uint32_t base, eax, signature[3];

for (base = 0x40000000; base < 0x40010000; base += 0x100) {
cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);

if (!memcmp(sig, signature, 12)
...
...


And the Xen detection code in OVMF:

for (mXenLeaf = 0x40000000; mXenLeaf < 0x40010000; mXenLeaf += 0x100) {
AsmCpuid (mXenLeaf,
NULL,
(UINT32 *) &Signature[0],
(UINT32 *) &Signature[4],
(UINT32 *) &Signature[8]);

if (!AsciiStrCmp ((CHAR8 *) Signature, "XenVMMXenVMM")) {
return TRUE;

The above functions are doing a loop-test.

The kernel patch also mentions about the loop-test :
https://lore.kernel.org/kvm/51FF1E26.6010707@redhat.com/t/

This patch introduce hypervisor_cpuid_base() which loop test the hypervisor
existence function until the signature match and check the number of leaves if
required. This could be used by Xen/KVM guest to detect the existence of
hypervisor.

The above patches/functions don't have any additonal documentation for
why are they doing the loop-test ?

I don't want to miss any functionality, hence i am reusing the same
loop-test code.

Thanks,
Ashish

+ AsmCpuid (
+ mKvmLeaf,
+ NULL,
+ (UINT32 *) &Signature[0],
+ (UINT32 *) &Signature[4],
+ (UINT32 *) &Signature[8]);
+
+ if (AsciiStrCmp (Signature, "KVMKVMKVM") == 0) {
+ DEBUG ((
+ DEBUG_INFO,
+ "%a: KVM Detected, signature = %a\n",
+ __FUNCTION__,
+ Signature
+ ));
+
+ RegEax = mKvmLeaf + 1;
+ RegEcx = 0;
+ AsmCpuid (mKvmLeaf + 1, &RegEax, &RegEbx, &RegEcx, &RegEdx);
+ if ((RegEax & KVM_FEATURE_MIGRATION_CONTROL) != 0) {
+ DEBUG ((
+ DEBUG_INFO,
+ "%a: SEV Live Migration feature supported\n",
+ __FUNCTION__
+ ));
+
+ return TRUE;
+ }
+ }
+ }
+
+ return FALSE;
+}