Re: [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Mon, May 30, 2022, Chao Peng wrote:
> On Mon, May 23, 2022 at 03:22:32PM +0000, Sean Christopherson wrote:
> > Actually, if the semantics are that userspace declares memory as private, then we
> > can reuse KVM_MEMORY_ENCRYPT_REG_REGION and KVM_MEMORY_ENCRYPT_UNREG_REGION.  It'd
> > be a little gross because we'd need to slightly redefine the semantics for TDX, SNP,
> > and software-protected VM types, e.g. the ioctls() currently require a pre-exisitng
> > memslot.  But I think it'd work...
> 
> These existing ioctls looks good for TDX and probably SNP as well. For
> softrware-protected VM types, it may not be enough. Maybe for the first
> step we can reuse this for all hardware based solutions and invent new
> interface when software-protected solution gets really supported.
> 
> There is semantics difference for fd-based private memory. Current above
> two ioctls() use userspace addreess(hva) while for fd-based it should be
> fd+offset, and probably it's better to use gpa in this case. Then we
> will need change existing semantics and break backward-compatibility.

My thought was to keep the existing semantics for VMs with type==0, i.e. SEV and
SEV-ES VMs.  It's a bit gross, but the pinning behavior is a dead end for SNP and
TDX, so it effectively needs to be deprecated anyways.  I'm definitely not opposed
to a new ioctl if Paolo or others think this is too awful, but burning an ioctl
for this seems wasteful.

Then generic KVM can do something like:

	case KVM_MEMORY_ENCRYPT_REG_REGION:
	case KVM_MEMORY_ENCRYPT_UNREG_REGION:
		struct kvm_enc_region region;

		if (!kvm_arch_vm_supports_private_memslots(kvm))
			goto arch_vm_ioctl;

		r = -EFAULT;
		if (copy_from_user(&region, argp, sizeof(region)))
			goto out;

		r = kvm_set_encrypted_region(ioctl, &region);
		break;
	default:
arch_vm_ioctl:
		r = kvm_arch_vm_ioctl(filp, ioctl, arg);


where common KVM provides

  __weak void kvm_arch_vm_supports_private_memslots(struct kvm *kvm)
  {
	return false;
  }

and x86 overrides that to

  bool kvm_arch_vm_supports_private_memslots(struct kvm *kvm)
  {
  	/* I can't remember what we decided on calling type '0' VMs. */
	return !!kvm->vm_type;
  }

and if someone ever wants to enable private memslot for SEV/SEV-ES guests we can
always add a capability or even a new VM type.

pKVM on arm can then obviously implement kvm_arch_vm_supports_private_memslots()
to grab whatever identifies a pKVM VM.