The Go Programming Language For Bare-Metal ARMv7a

These are my modifications to Go that enable it to run bare-metal on armv7a SOCs. The basic OS primitives that Go relies on have been re-implemented in Go and Plan 9 assembly inside the runtime package.

This repo tracks the master branch of the main go repo at https://github.com/golang/go. One day this code might even be part of Go but, until then, it will sit here.

The modified runtime is also an integral part of G.E.R.T, the Golang Embedded RunTime. Check it out for working examples.

https://github.com/ycoroneos/G.E.R.T

Modifications To Go

The majority of the runtime modifications are inside src/runtime. Each of the files below have comments in them which distinguish the different sections and explain the functions.

Bootloaders

Uboot sets up the initial device clocks and chainloads the GERT bootloader, which is written in C. The GERT bootloader prepares a stack before loading the GERT ELF and jumping into it. GERT is linked and loaded at 0x1100_0000. The C bootloader is linked and loaded at 0x5000_0000. These addresses are due to the memory map of the iMX6.

GERT Boot Process

The GERT entry point is main.Entry inside the main package. It's easy to tell Go about a new entry point and link address in a Makefile:

GOLINKFLAGS := "-T <link_address> -E main.Entry"

main.Entry sets runtime.Armhackmode=True and calls runtime.Runtime_main(), which continues the Go boot process. Since Armhackmode=True, the normal Go boot process takes a few detours to setup threading, virtual memory, and context switching.

Virtual Memory

GERT uses a single L1 page table with 1MB pages in it. This is because GERT runs in a single address space. Go's own memory safety and isolation primitives means that multiple address spaces with fine page granularity are unnecessary. The reason the MMU is even on is to deal with gaps in the physical memory space near address 0x0. Additionally, the Go runtime sometimes requests a fixed page mapping which can't exist in the physical space.

Syscalls

Calls to SWI have been converted to trap_debug. All relevent linux syscalls that Go relies on have been re-implemented in Go inside trap_debug inside sys_linux_arm.s. trap_debug lives inside gert_arm.go. When the Go runtime executes a syscall, it actually redirects execution back into Go. Sometimes syscalls can be blocking (such as yield or futex) so the modified runtime maintains a threadlist and trapframes to switch between.

Time

GERT uses the 64bit ARM global timer as its primary time source. Each tick is approximately 2ns and it won't roll over for a few thousand years. Rounding error causes inaccuracy to build up though.

Interrupts

GERT can service interrupts at any time, even when Go's garbage colector has stopped the world to scan stacks. This is because the GERT interrupt handler is effectively a secret to the rest of the runtime. In the interrupt handler there can be no blocking operations like channel loads, or heap allocations with make(). This isn't very bad, just toggle a global flag for other goroutines to use. You can still interact with peripherals during an interrupt because those are just memory reads/writes.

Modification

If the iMX6Quad is not your SOC then you can still use this code but you will have to write some drivers to port it.

Write a new UART driver in src/runtime/write_err.go

Modify map_kernel() in src/runtime/gert_arm.go to reflect your memory map

Modify mp_init() in src/runtime/gert_arm.go as well as boot_any in src/runtime/gertasm_arm.s to boot your other cpus