[feature/opensbi_integration] Starting firmware

[alarmfox]

Firmware init

This PR starts creating the firmware. The main idea is to use OpenSBI to avoid rewriting initialization code for platform and focusing our firmware in tsm_driver part. OpenSBI allows also to create domains using a device tree.

I found this closed issues on opensbi. Leaving there for reference:

• Start the OpenSBI Payload in m-mode ? riscv-software-src/opensbi#317
• How to enter M-mode? riscv-software-src/opensbi#310

OpenSBI can launch firmware in 3 ways, but they cannot be used in our use case because they are executed with S or HS priviledge mode. According to CoVE specification, Shadowfax must run in M mode to be a compliant tsm_driver.

Fortunately, OpenSBI can be used from an external firmware (shadowfax itself) and linked as a static library. According to the documentation, we need to initialize harts using sbi_init() and setup trap handler with sbi_trap_handler() with the following requirements:

1. The RISC-V MSCRATCH CSR must point to a valid OpenSBI scratch space
   (i.e. a struct sbi_scratch instance).
2. The RISC-V SP register (i.e. the stack pointer) must be set per-HART
   pointing to distinct non-overlapping stacks.
   The most important functions from an external firmware or bootloader
   perspective are sbi_init() and sbi_trap_handler().

In addition to the above constraints, the external firmware or bootloader must
ensure that interrupts are disabled in the MSTATUS and MIE CSRs when calling
the functions sbi_init() and sbi_trap_handler().

external firmwares or bootloaders can be more conservative by
forwarding all traps and interrupts to sbi_trap_handler().

We can start working from here with some assumptions (ie. only 1 hart with id 0). We can take as reference the file firmware/fw_base.S in OpenSBI source code in which hart and the platform are initialized.

Platform vs non platform library

OpenSBI exposes 2 different static library:

• libsbi.a: platform agnostic library which must be integrated with specific initialization hooks. In this case, we should provide our custom device tree;
• libplatsbi.a: which performs also init hooks for specific platform (the ones supported by the opensbi spec). The library will be different based on the PLATFORM flag specified when compiling.

In our case, we could use the generic platform which can be used to test against the virt machine in QEMU.

Linkerscript

src/init.ld provides a basic linkerscript taken from examples/helloworld. We need to evaluate if other sections are needed and to need to parametrize some variables like start address and end address to make it compatible with the target platform.

Running the example

For simplicity, the example is in assembly. The goal is to setup the platform in a minimal way:

• sbi_init for hart 0
• setup sbi_trap_handler
• jump to an helloworld function

First, clone the repository and checkout the branch:

git clone --depth 1 -b feature/opensbi_integration git@github.com:Granp4sso/shadowfax.git

Now, get OpenSBI:

curl -fsSLO https://github.com/riscv-software-src/opensbi/archive/refs/tags/v1.6.tar.gz
tar xvf v1.6.tar.gz
cd opensbi-1.6

Compile and install OpenSBI using the cross-compile toolchain (substitute the riscv if needed and the path to shadowfax accordingly) for the generic platform:

make -j`nproc` PLATFORM=generic CROSS_COMPILE=riscv64-linux-gnu-
make I=../shadowfax install

Run "shadowfax firmware" using:

cd ../shadowfax/
make CROSS_COMPILE=riscv64-linux-gnu- run

The output should be:

> make CROSS_COMPILE=riscv64-linux-gnu- run
riscv64-linux-musl-gcc -Wall -Wextra -march=rv64gh -mabi=lp64 -Iinclude/ -c -o init.o init.S
riscv64-linux-musl-ld  -static -o shadowfax -T init.ld init.o ../lib64/lp64/libsbi.a
Press (ctrl + a) and then x to quit
qemu-system-riscv64 -nographic -machine virt -bios shadowfax
hello