Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend specific

[Sean Christopherson]

On Thu, May 30, 2024, Kai Huang wrote:
> On Wed, 2024-05-29 at 16:15 -0700, Sean Christopherson wrote:
> > In the unlikely event there is a legitimate reason for max_vcpus_per_td being
> > less than KVM's minimum, then we can update KVM's minimum as needed.  But AFAICT,
> > that's purely theoretical at this point, i.e. this is all much ado about nothing.
> 
> I am afraid we already have a legitimate case: TD partitioning.  Isaku
> told me the 'max_vcpus_per_td' is lowed to 512 for the modules with TD
> partitioning supported.  And again this is static, i.e., doesn't require
> TD partitioning to be opt-in to low to 512.

So what's Intel's plan for use cases that creates TDs with >512 vCPUs?

> So AFAICT this isn't a theoretical thing now.
> 
> Also, I want to say I was wrong about "MAX_VCPUS" in the TD_PARAMS is part
> of attestation.  It is not.  TDREPORT dosen't include the "MAX_VCPUS", and
> it is not involved in the calculation of the measurement of the guest.
> 
> Given "MAX_VCPUS" is not part of attestation, I think there's no need to
> allow user to change kvm->max_vcpus by enabling KVM_ENABLE_CAP ioctl() for
> KVM_CAP_MAX_VCPUS.

Sure, but KVM would still need to advertise the reduced value for KVM_CAP_MAX_VCPUS
when queried via KVM_CHECK_EXTENSION.  And userspace needs to be conditioned to
do a VM-scoped check, not a system-scoped check.

> So we could just once for all adjust kvm->max_vcpus for TDX in the
> tdx_vm_init() for TDX guest:
> 
> 	kvm->max_vcpus = min(kvm->max_vcpus, tdx_info->max_vcpus_per_td);
> 
> AFAICT no other change is needed.
> 
> And in KVM_TDX_VM_INIT (where TDH.MNG.INIT is done) we can just use kvm-
> >max_vcpus to fill the "MAX_VCPUS" in TD_PARAMS.