Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 16, 2022, Gupta, Pankaj wrote:
> 
> > > > Actually the current version allows you to delay the allocation to a
> > > > later time (e.g. page fault time) if you don't call fallocate() on the
> > > > private fd. fallocate() is necessary in previous versions because we
> > > > treat the existense in the fd as 'private' but in this version we track
> > > > private/shared info in KVM so we don't rely on that fact from memory
> > > > backstores.
> > > 
> > > Does this also mean reservation of guest physical memory with secure
> > > processor (both for SEV-SNP & TDX) will also happen at page fault time?
> > > 
> > > Do we plan to keep it this way?
> > 
> > If you are talking about accepting memory by the guest, it is initiated by
> > the guest and has nothing to do with page fault time vs fallocate()
> > allocation of host memory. I mean acceptance happens after host memory
> > allocation but they are not in lockstep, acceptance can happen much later.
> 
> No, I meant reserving guest physical memory range from hypervisor e.g with
> RMPUpdate for SEV-SNP or equivalent at TDX side (PAMTs?).

As proposed, RMP/PAMT updates will occur in the fault path, i.e. there is no way
for userspace to pre-map guest memory.

I think the best approach is to turn KVM_TDX_INIT_MEM_REGION into a generic
vCPU-scoped ioctl() that allows userspace to pre-map guest memory.  Supporting
initializing guest private memory with a source page can be implemented via a
flag.  That also gives KVM line of sight to in-place "conversion", e.g. another
flag could be added to say that the dest is also the source.

The TDX and SNP restrictions would then become addition restrictions on when
initializing with a source is allowed (and VMs that don't have guest private
memory wouldn't allow the flag at all).