Re: [PATCH v4 4/4] OvmfPkg/PlatformDxe: Add support for SEV live migration.

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

Hello Tom,

On Tue, Jun 22, 2021 at 06:06:24PM -0500, Tom Lendacky wrote:
> > +
> > +/**
> > +  Figures out if we are running inside KVM HVM and
> > +  KVM HVM supports SEV Live Migration feature.
> > +
> > +  @retval TRUE   KVM was detected and Live Migration supported
> > +  @retval FALSE  KVM was not detected or Live Migration not supported
> > +
> > +**/
> > +BOOLEAN
> > +KvmDetectSevLiveMigrationFeature(
> > +  VOID
> > +  )
> > +{
> > +  UINT8 Signature[13];
> > +  UINT32 mKvmLeaf = 0;
> > +  UINT32 RegEax, RegEbx, RegEcx, RegEdx;
> > +
> > +  Signature[12] = '\0';
> > +  for (mKvmLeaf = 0x40000000; mKvmLeaf < 0x40010000; mKvmLeaf += 0x100) {
> 
> What's the reason for the loop? I would think that just checking
> 0x40000000 would be enough, so a comment seems to be warranted.
> 

0x40000000 leaf is the hypervisor CPUID information leaf, so probably
just checking 0x40000000 should be enough.

But i see that other hypervisor detection functions like XenDetect() 
do a loop test on the hypervisor existence function until the
signature match, is there a specific reason for that ?

Is this for some kind of support for another/multiple hypervisors ?

> > +    AsmCpuid (mKvmLeaf,
> > +              NULL,
> > +              (UINT32 *) &Signature[0],
> > +              (UINT32 *) &Signature[4],
> > +              (UINT32 *) &Signature[8]);
> > +
> > +    if (!AsciiStrCmp ((CHAR8 *) Signature, "KVMKVMKVM\0\0\0")) {
> > +      DEBUG ((
> > +        DEBUG_ERROR,
> 
> DEBUG_INFO, it doesn't seem like an error.
> 

Ok.

> > +        "%a: KVM Detected, signature = %s\n",
> > +        __FUNCTION__,
> > +        Signature
> > +        ));
> > +> +      RegEax = 0x40000001;
> 
> Should this be mKvmLeaf + 1? It is confusing that you may check 0x40000100
> and then not do 0x40000101.
> 

Yes, it should be mKvmLeaf + 1, assuming the loop above is being used.

> > +      RegEcx = 0;
> > +      AsmCpuid (0x40000001, &RegEax, &RegEbx, &RegEcx, &RegEdx);
> > +      if (RegEax & (1 << KVM_FEATURE_MIGRATION_CONTROL)) {
> 

Thanks,
Ashish