[GSoC] Better RISC-V support, week #3

Last week, after updating GCC (by applying Iru Cai’s patch) and commenting out uses of outdated instructions and CSRs (most notably eret and the HTIF CSRs), I noticed that coreboot crashed when it tried to access any global variables. This was because the coreboot build system thought coreboot would live near the start of the address space.

I found spike-riscv/memlayout.ld, and adjusted the starting offset. But then I got a linker error:

build/bootblock/arch/riscv/rom_media.o: In function `boot_device_ro': [...]/src/arch/riscv/rom_media.c:26:(.text.boot_device_ro+0x0): relocation truncated to fit: _RISCV_HI20 against `.LANCHOR0'

I played around with the start address and noticed that addresses below 0x78000000 worked, but if I chose an address that was too close to 0x80000000, it broke. This is, in fact, because pointers to global variables were determined with an instruction sequence like lui a0, 0xNNNNN; addi a0, a0, 0xNNN. On 32-bit RISC-V, the LUI instruction loads its argument into the upper 20 bits of a register, and ADDI adds a 12-bit number. On a 64-bit RISC-V system, lui a0, 0x80000 loads 0xffffffff80000000 into a0, because the number is sign extended.

After disassembling some .o files of coreboot and the RISC-V proxy kernel, I finally noticed that I had to use the -mcmodel=medany compiler option, which makes data accesses pc-relative.

Now that coreboot finally ran and could access its data section, I finished debugging the UART block that I promised last week. Coreboot can now print stuff, but it stops running pretty soon.

Plans for this week

This week I will debug why coreboot hangs, and will hopefully get it to boot until the “Payload not found” line again, which worked with an older version of Spike.

Also, Ron Minnich will be giving a talk about coreboot on RISC-V at the coreboot convention in San Francisco, in a few hours.