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

[Andy Lutomirski <luto@kernel.org>]

>>
>> 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