I am referring to both the design, and the independent, and auditable manufacture of the CPU. It should be noted that such a CPU needn’t fully compete with modern ARM, Intel, AMD, etc. CPUs, but it would be an incredible boon to have a fully trustworthy piece of hardware, even if it is considerably lower in it’s strength. For specifics, let’s say a CPU that could run a lightweight Linux distro at a “tolerable” speed.
Creating the designs for the CPU, of course while still difficult, is, most likely, the most feesbile aspect – I presume it would “just” consist of writing the Verilog, or some other hardware description language to describe the CPU’s function. The manufacture, however, is a substantial obstacle. Modern photolithography is, quite litterally, at the very forefront of human technological creation. I am just hoping that turning back the clock perhaps 20 years on the technological complexity might reduce the barrier to entry.
It’s all a question of how much money you have access to. OpenCores has a bunch of HDL processors that can serve as a base design that could be turned into an ASIC and sent to a fab company. From what I’ve heard it costs on the order of a million USD for a run of wafers. You will own the design fully, there’s very little chance they would insert anything onto the wafer you didn’t ask for. Difficult to truly verify though: you hand the vendor instructions like “put a D flip flop here” and they turn that into all of the process step lithography masks. You can shop that same design to multiple fab vendors but due to IP differences it might require tweaking per vendor.
Maybe if you work with a university’s clean room you could do it for less, but they won’t have a library of drop-in IP blocks for transistors, clock generators, PLLs, high speed I/O (eg pci lanes), memory, etc so now you have to develop those from scratch.
Similar concept if you were able to acquire some used equipment: all the process engineering needed to make a functioning transistor now falls on your project, as does sourcing the chemicals, getting the masks fabricated, handing the hazardous waste, maintaining a clean room, the list goes on.
All these hurdles are achievable of course, but that requires some serious engineering skills, and those folks tend to cost a lot of money.
The short cut to all of this is, if you aren’t trying to sell millions of these things, just implement the CPU in an fpga and call it a day. The newest mid-range FPGAs (~ $1k range) can readily reach 500MHz and have 8ish lanes of PCIe 5.0. The security mechanisms can also be used to prevent any other firmware other than yours from running on the chip (encrypted firmware plus an authentication layer).