add initial nuttx-rv64ilp32

Signed-off-by: Yanfeng Liu <[email protected]>

docs/README.md

@@ -1,4 +1,4 @@
 Some notes on NuttX
 
 How [clean](leakless-ostest) is NuttX?
-
+How does [NuttX with rv64ilp32](nuttx-rv64ilp32) feel like?

docs/nuttx-rv64ilp32.md

@@ -0,0 +1,68 @@
+# NuttX and RV64ILP32
+
+This page shares information about NuttX built with the `rv64ilp32` toolchain.
+
+## The Problem
+
+Many AIoT chips using RISC-V on the market are following 64 bit profiles but with small sized memory. They can run 64-bit system software but resource usage isn't optimized, as almost all 64 bit pointers only have their lower 32-bit carry valid information.
+
+So the industry has attempted to run software using 32-bit pointers to reduce memory footprint while improve cache performance, while at same time enjoy the 64-bit CPU processing power, with the help of a toolchain that supports the `rv64ilp32` ABI, read this [LWN page](https://lwn.net/Articles/951187) for background information.
+
+Though the requirements for `rv64ilp32` from Linux looks more urgent, but essentially RTOS is of no exception as efficiency is also key for RTOS.
+
+So can we have `nuttx-rv64ilp32` as well?
+
+## The toolchain
+
+After downloading the [RuyiSDK rv64ilp32 toolchain](https://github.com/ruyisdk/riscv-gnu-toolchain-rv64ilp32/releases/tag/2024.05.23), playing the `hello world` on the QEMU in the toolchain and raising questions to experts to learn how it works.
+
+Here are the major findings:
+
+- The `rv64ilp32` program is of ELF-32 class, but with flag 0x25;
+- The `qemu-riscv64ilp32` in the toolchain can the `rv64ilp32` program by supporting the new program format and ignoring the higher 32-bit addresses generated by the compiler;
+- The toolchain uses `-march=rv64`, `-mabi=ilp32` to generate rv64ilp32 binaries;
+- The compiler has `__riscv_xlen_t==64` and `sizeof(void*)==32` at the same time;
+
+
+Requirements for the RV64 chips is still to be checked.
+
+## Building NuttX
+
+With toolchain knowledge and a few tweaks in the following steps:
+
+- Add `rv64ilp32` toolchain in `arch/risc-v` Kconfig and Toolchain.cmake to support generation of nuttx-rv64ilp32 binary.
+- Add a type for register width data in `arch/risc-v` layer, with some tweaking of the code base to support both existing 64-bit and 32-bit ABIs and the new rv64ilp32 ABI.
+
+Thanks to NuttX's well designed structure, the generated nuttx image can boot smoothly on the QEMU from the toolchain, though there are still some cleanups left behind. The `ostest` check passed smoothly as well.
+
+## Comparison
+
+Here are initial comparisons between normal `rv64lp64` and the `rv64ilp32`, the baseline configuration is `rv-virt/nsh64`.
+
+Static footprints:
+
+```
+$ cd /tmp && size nuttx-*
+   text    data     bss     dec     hex filename
+ 176861     397   10256  187514   2dc7a nuttx-ilp32
+ 173171     681   12416  186268   2d79c nuttx-lp64
+```
+
+Dynamic footprints:
+
+```
+nsh> cat /proc/version
+NuttX version 12.4.0 77abbe5a43-dirty Jun  6 2024 16:22:56 rv-virt/nsh64ilp32
+nsh> free
+                 total       used       free    maxused    maxfree  nused  nfree
+      Umem:   33364764       8068   33356696       8052   33356664     21      2
+
+nsh> cat /proc/version
+NuttX version 12.4.0 5ce64a46a5-dirty Jun  6 2024 16:23:33 rv-virt/nsh64
+nsh> free
+                 total       used       free    maxused    maxfree  nused  nfree
+      Umem:   33366008      10344   33355664      10312   33355616     21      2
+```
+
+We can see that the code size if `rv64ilp32` is about 2.13% bigger, and data memory reduced 18% or 22% for static or heap memory.
+