Re: [PATCH v5 07/17] KVM: x86: add fields to struct kvm_arch for CoCo features

[Sean Christopherson]

On Tue, May 07, 2024, Rick P Edgecombe wrote:
> On Mon, 2024-04-08 at 18:21 -0700, Sean Christopherson wrote:
> > > - Give other name for this check like zap_from_leafs (or better name?)
> > >    The implementation is same to kvm_gfn_shared_mask() with comment.
> > >    - Or we can add a boolean variable to struct kvm
> > 
> > If we _don't_ hardcode the behavior, a per-memslot flag or a per-VM
> > capability (and thus boolean) is likely the way to go.  My off-the-cuff
> > vote is probably for a per-memslot flag.
> 
> Hi Sean,
> 
> Can you elaborate on the reason for a per-memslot flag? We are discussing this
> design point internally, and also the intersection with the previous attempts to
> do something similar with a per-vm flag[0].
> 
> I'm wondering if the intention is to try to make a memslot flag, so it can be
> expanded for the normal VM usage.

Sure, I'll go with that answer.  Like I said, off-the-cuff.

There's no concrete motiviation, it's more that _if_ we're going to expose a knob
to userspace, then I'd prefer to make it as precise as possible to minimize the
changes of KVM ending up back in ABI hell again.

> Because the discussion on the original attempts, it seems safer to keep this
> behavior more limited (TDX only) for now.  And for TDX's usage a struct kvm
> bool fits best because all memslots need to be set to zap_leafs_only = true,
> anyway.

No they don't.  They might be set that way in practice for QEMU, but it's not
strictly required.  E.g. nothing would prevent a VMM from exposing a shared-only
memslot to a guest.  The memslots that burned KVM the first time around were
related to VFIO devices, and I wouldn't put it past someone to be crazy enough
to expose an passhtrough an untrusted device to a TDX guest.

> It's simpler for userspace, and less possible situations to worry about for KVM.
> 
> [0] https://lore.kernel.org/kvm/20200703025047.13987-1-sean.j.christopherson@xxxxxxxxx/