[GSoC] Better RISC-V support, week #6/7/8/9

Since I haven’t posted an update of my coreboot-on-RISC-V work in a while, this will be a slightly longer post.

Week 6

In week 6, I started documenting how to build and boot coreboot on RISC-V, in the coreboot wiki.
It is now a bit outdated, because we’re moving away from using bbl to boot Linux.

Week 7

I wrote some patches: I removed code that used the old Host-Target Interface (HTIF), because it’s deprecated. I submitted an improved version of my workaround for the bug that causes Spike to only execute 5000 instructions in some cases. I informed the coreboot resource management subsystem about the position of the RAM in physical address space, so that the program loader wouldn’t refuse to load segments into RAM. I submitted two patches to fix compiler errors with the new toolchain.

Meanwhile, there were some good news in the RISC-V world:

Week 8

I submitted a few more patches and started to explore the Nexys4 board. The precompiled bitstream and kernel from the lowRISC version 0.3 tutorial worked without any problems, and after a few days and some help from the lowRISC mailing list, I was able to recompile the lowRISC bitstream.

This week

I discussed the choice of boot medium with the lowRISC developers, and they agreed that a memory-mapped flash would be useful. Once it is implemented, I
can start porting coreboot to lowRISC on the Nexys 4 DDR board. Luckily the Nexys4 already has large enough flash to use for this purpose.

My mentors and I agreed that the switch from machine mode to supervisor mode should be left completely to the payload.

What’s next?

I will continue to work on running coreboot on the Spike emulator. Currently I’m facing the following problems and tasks:

  • Linux, when compiled to an ELF file (vmlinux) specifies that it wants to be loaded at the physical address 0x0 and at the virtual address 0xffffffff80000000. Since coreboot’s ELF loading code only looks at the physical address, it refuses to load Linux, since RAM starts at 0x80000000 on RISC-V.
  • Low level platform information (most importantly the memory layout) is passed to the firmware (coreboot in this case) as a configuration string, which is dynamically generated by the emulator, in the case of Spike. I still need to implemented a parser for this format, so coreboot can know how much memory is available.
  • The RISC-V Privileged Architecture Specification 1.9 specifies that there shall be a page at the top of the virtual address space where the operating system can call a few functions exposed by the firmware (this is the Supervisor Binary Interface).