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/51ff1e26.6010...@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;
> > +}
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