Dynamic PAMT has made it for Linux 7.4! Dynamic PAMT is a new feature for reducing memory overhead on Intel TDX servers where there may not be many concurrent virtual machines occupying a majority of the system. In today's world with high memory prices and tight availability, Dynamic PAMT provides nice memory overhead reduction for modern Intel Xeon servers.
Dynamic PAMT is a new Intel Trust Domain Extensions (TDX) feature to reduce the amount of memory pre-allocated for metadata and is most impactful for Xeon servers where they aren't over saturated with confidential computing virtual machines. Now the Physical Address Metadata Table (PAMT) will grow dynamically as needed for CoCo VMs rather than carving out a static ~0.4% of the total system RAM. With today's high-end Intel Xeon servers with lots of RAM, the ~0.4% default can end up eating a lot of RAM that isn't necessarily being used if there aren't many VMs active.
Intel Linux engineer Dave Hansen explained in the TDX pull request for Linux 7.4:
"First, TDX host systems currently consume ~0.4% of physical memory for their own memory metadata. This is all-or-nothing; you either pay the 0.4% or can not use TDX. A new TDX module feature (Dynamic PAMT) allows this metadata to be allocated for 2MB of host memory at a time. This, of course, saves precious DRAM when TDX utilization is low on a host.Second, the TDX ABI has fixed-size "quotes" that are passed around during attestation operations. These 128k buffers are far too small for post-quantum cryptography where the quotes grow substantially. The TDX module now has the ability to enumerate the sizes of the quote buffers. Plumb the new size enumeration into the kernel."
The pull request was honored and now part of Linux 7.4 Git.