x86/virt/tdx: Add placeholder to construct TDMRs to cover all TDX memory regions

After the kernel selects all TDX-usable memory regions, the kernel needs
to pass those regions to the TDX module via data structure "TD Memory
Region" (TDMR).

Add a placeholder to construct a list of TDMRs (in multiple steps) to
cover all TDX-usable memory regions.

=== Long Version ===

TDX provides increased levels of memory confidentiality and integrity.
This requires special hardware support for features like memory
encryption and storage of memory integrity checksums.  Not all memory
satisfies these requirements.

As a result, TDX introduced the concept of a "Convertible Memory Region"
(CMR).  During boot, the firmware builds a list of all of the memory
ranges which can provide the TDX security guarantees.  The list of these
ranges is available to the kernel by querying the TDX module.

The TDX architecture needs additional metadata to record things like
which TD guest "owns" a given page of memory.  This metadata essentially
serves as the 'struct page' for the TDX module.  The space for this
metadata is not reserved by the hardware up front and must be allocated
by the kernel and given to the TDX module.

Since this metadata consumes space, the VMM can choose whether or not to
allocate it for a given area of convertible memory.  If it chooses not
to, the memory cannot receive TDX protections and can not be used by TDX
guests as private memory.

For every memory region that the VMM wants to use as TDX memory, it sets
up a "TD Memory Region" (TDMR).  Each TDMR represents a physically
contiguous convertible range and must also have its own physically
contiguous metadata table, referred to as a Physical Address Metadata
Table (PAMT), to track status for each page in the TDMR range.

Unlike a CMR, each TDMR requires 1G granularity and alignment.  To
support physical RAM areas that don't meet those strict requirements,
each TDMR permits a number of internal "reserved areas" which can be
placed over memory holes.  If PAMT metadata is placed within a TDMR it
must be covered by one of these reserved areas.

Let's summarize the concepts:

 CMR - Firmware-enumerated physical ranges that support TDX.  CMRs are
       4K aligned.
TDMR - Physical address range which is chosen by the kernel to support
       TDX.  1G granularity and alignment required.  Each TDMR has
       reserved areas where TDX memory holes and overlapping PAMTs can
       be represented.
PAMT - Physically contiguous TDX metadata.  One table for each page size
       per TDMR.  Roughly 1/256th of TDMR in size.  256G TDMR = ~1G
       PAMT.

As one step of initializing the TDX module, the kernel configures
TDX-usable memory regions by passing a list of TDMRs to the TDX module.

Constructing the list of TDMRs consists below steps:

1) Fill out TDMRs to cover all memory regions that the TDX module will
   use for TD memory.
2) Allocate and set up PAMT for each TDMR.
3) Designate reserved areas for each TDMR.

Add a placeholder to construct TDMRs to do the above steps.  To keep
things simple, just allocate enough space to hold maximum number of
TDMRs up front.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/all/20231208170740.53979-9-dave.hansen%40intel.com