Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

quoted

In principle, you could actually initialize a TDX guest with all of its
memory shared and all of it mapped in the host IOMMU.  When a guest
turns some pages private, user code could punch a hole in the memslot,
allocate private memory at that address, but leave the shared backing
store in place and still mapped in the host IOMMU.  The result would be
that guest-initiated DMA to the previously shared address would actually
work but would hit pages that are invisible to the guest.  And a whole
bunch of memory would be waste, but the whole system should stll work.

Do you mean to let VFIO & IOMMU to treat all guest memory as shared first,
and then just allocate the private pages in another backing store? I guess
that could work, but with the cost of allocating roughly 2x physical pages
of the guest RAM size. After all, the shared pages shall be only a small
part of guest memory.

Yes.

My point is that I don't think there should be any particular danger in
leaving the VFIO code alone as part of TDX enablement.  The code ought
to *work* even if it will be wildly inefficient.  If someone cares to
make it work better, they're welcome to do so.

--Andy