KVM: guest_memfd: Make private exactly what can be mapped on page fault

When calling into arch code to make the underlying memory private, i.e. to
assign memory to the VM in SNP's RMP table, assign/convert *exactly* the
range of memory that can be mapped into the guest for the current page
fault, instead of aggressively converting/assigning the entire folio.  For
SNP, the mapping size in the stage-2 page tables (Nested Page Tables, NPT)
must be at least the size of the corresponding RMP entry, e.g. assigning a
2MiB mapping in the RMP when it can only be mapped at 4KiB granualarity
will ultimate result in another page fault (#NPF for SNP) to "smash" the
RMP down to the correct mapping size.

Assigning the entire folio was necessary back when guest_memfd tracked
preparedness, which was done on a per-folio basis.  At the time, it made
sense to do per-folio tracking/preparation, because tracking per-folio
meant guest_memfd didn't need to add a separate data structure to track
that information, and doing per-folio tracking only works if the entire
folio is prepared (or not).

Now that guest_memfd no longer does preparation tracking (see commit
8622ef05709f ("KVM: guest_memfd: Remove preparation tracking")), in favor
having SNP query the RMP, per-folio preparation, i.e. per-folio conversions
to private, doesn't make any sense.

*If* SNP allowed the RMP size to be greater than the NPT size, then
per-folio conversion could theoretically provide marginal value, as it
would allow KVM to assign a hugepage in the RMP even if it can only be
mapped into the NPT with a smaller page, e.g. because of memslot alignment.
The documentation of that reasoning would be something like this:

  /*
   * If the memory is private from KVM's perspective, and hardware tracks
   * VM-assigned private memory in a dedicated data structure, i.e. not
   * in the stage-2 page tables, then call into arch code to assign the
   * entire folio to the guest.  Assigning the entire folio, e.g. instead
   * of only the memory being mapped into the guest, allows KVM to assign
   * an entire hugepage of memory in the out-of-band structure even if
   * KVM can only map a smaller page size into the MMU, e.g. because the
   * gmem hugepage is spread across multiple memslots.
   */

But even *if* a future SNP implementation supported that behavior, the
value added would be dubious, as having a huge folio that is fully private,
but can only be mapped at a smaller granularity, would be rare.  E.g. maybe
for memory at the top of lower DRAM that has holes for non-RAM assets?

So, convert/assign exactly what guest_memfd allows the caller to map to
simplify the guest_memfd code and provide a (super) minor performance
optimization for SNP.  E.g. once hugepage support comes along, guest_memfd
will only need a single flow to compute "how much memory can be assigned
and at what size".

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260723210811.72720-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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