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

[Chao Peng]

On Fri, Jun 10, 2022 at 04:14:21PM +0000, Sean Christopherson wrote:
> 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. 

Yes agreed.

> 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.

Yes, I also feel confortable if it's acceptable to reuse kvm_enc_region
to pass _gpa_ range for this new type.

> 
> 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;
>   }

I already had kvm_arch_private_mem_supported() introduced in patch-07
so that can be reused.

> 
> 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.