Re: [PATCH v6 1/6] OvmfPkg/BaseMemEncryptLib: Detect SEV live migration feature.

["Ashish Kalra" <ashish.kalra@amd.com>]

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;
> > +}