Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

[Chao Peng]

On Tue, Aug 02, 2022 at 04:38:55PM +0000, Sean Christopherson wrote:
> On Tue, Aug 02, 2022, Sean Christopherson wrote:
> > I think we should avoid UNMAPPABLE even on the KVM side of things for the 
> > core
> > memslots functionality and instead be very literal, e.g.
> > 
> >     KVM_HAS_FD_BASED_MEMSLOTS
> >     KVM_MEM_FD_VALID
> > 
> > We'll still need KVM_HAS_USER_UNMAPPABLE_MEMORY, but it won't be tied 
> > directly to
> > the memslot.  Decoupling the two thingis will require a bit of extra work, 
> > but the
> > code impact should be quite small, e.g. explicitly query and propagate
> > MEMFILE_F_USER_INACCESSIBLE to kvm_memory_slot to track if a memslot can be 
> > private.
> > And unless I'm missing something, it won't require an additional memslot 
> > flag.
> > The biggest oddity (if we don't also add KVM_MEM_PRIVATE) is that KVM would
> > effectively ignore the hva for fd-based memslots for VM types that don't 
> > support
> > private memory, i.e. userspace can't opt out of using the fd-based backing, 
> > but that
> > doesn't seem like a deal breaker.

I actually love this idea. I don't mind adding extra code for potential
usage other than confidential VMs if we can have a workable solution for
it.

> 
> Hrm, but basing private memory on top of a generic FD_VALID would effectively 
> require
> shared memory to use hva-based memslots for confidential VMs.  That'd yield a 
> very
> weird API, e.g. non-confidential VMs could be backed entirely by fd-based 
> memslots,
> but confidential VMs would be forced to use hva-based memslots.

It would work if we can treat userspace_addr as optional for
KVM_MEM_FD_VALID, e.g. userspace can opt in to decide whether needing
the mappable part or not for a regular VM and we can enforce KVM for
confidential VMs. But the u64 type of userspace_addr doesn't allow us to
express a 'null' value so sounds like we will end up needing another
flag anyway.

In concept, we could have three cofigurations here:
  1. hva-only: without any flag and use userspace_addr;
  2. fd-only:  another new flag is needed and use fd/offset;
  3. hva/fd mixed: both userspace_addr and fd/offset is effective.
     KVM_MEM_PRIVATE is a subset of it for confidential VMs. Not sure
     regular VM also wants this.

There is no direct relationship between unmappable and fd-based since
even fd-based can also be mappable for regular VM?

> 
> Ignore this idea for now.  If there's an actual use case for generic fd-based 
> memory
> then we'll want a separate flag, fd, and offset, i.e. that support could be 
> added
> independent of KVM_MEM_PRIVATE.

If we ignore this idea now (which I'm also fine), do you still think we
need change KVM_MEM_PRIVATE to KVM_MEM_USER_UNMAPPBLE?

Thanks,
Chao