riscv-from-scratch
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twilco
Rom memory overlap error
Hey, thanks for a brilliantly written tutorial. Impressive clarity!
I am trying to make it work with qemu-riscv64 and riscv64-elf-* tools from archlinux. It compiles fine, but when I run, I get the following error:
$ qemu-system-riscv64 -machine virt -m 128M -gdb tcp::1234 -S -kernel a.out
VNC server running on ::1:5900
rom: requested regions overlap (rom /usr/share/qemu/opensbi-riscv64-virt-fw_jump.bin. free=0x0000000080000054, addr=0x0000000080000000)
qemu-system-riscv64: rom check and register reset failed
I've reduced the main code to just int main { return 0; }, which disassembles to:
$ riscv64-elf-objdump -d a.out
a.out: file format elf64-littleriscv
Disassembly of section .init:
0000000080000000 <_start>:
80000000: 00002197 auipc gp,0x2
80000004: 88418193 addi gp,gp,-1916 # 80001884 <__global_pointer$>
80000008: 08000117 auipc sp,0x8000
8000000c: ff810113 addi sp,sp,-8 # 88000000 <__stack_top>
80000010: 00010433 add s0,sp,zero
80000014: 0040006f j 80000018 <main>
Disassembly of section .text:
0000000080000018 <main>:
80000018: 1101 addi sp,sp,-32
8000001a: ec22 sd s0,24(sp)
8000001c: 1000 addi s0,sp,32
8000001e: 4785 li a5,1
80000020: fef42623 sw a5,-20(s0)
80000024: 4789 li a5,2
80000026: fef42423 sw a5,-24(s0)
8000002a: fec42703 lw a4,-20(s0)
8000002e: fe842783 lw a5,-24(s0)
80000032: 9fb9 addw a5,a5,a4
80000034: fef42223 sw a5,-28(s0)
80000038: fe442783 lw a5,-28(s0)
8000003c: 2785 addiw a5,a5,1
8000003e: 2781 sext.w a5,a5
80000040: fe442703 lw a4,-28(s0)
80000044: 9fb9 addw a5,a5,a4
80000046: fef42223 sw a5,-28(s0)
8000004a: 4781 li a5,0
8000004c: 853e mv a0,a5
8000004e: 6462 ld s0,24(sp)
80000050: 6105 addi sp,sp,32
80000052: 8082 ret
[lash@tranquillo riscv]$ riscv64-elf-objdump -D a.out
a.out: file format elf64-littleriscv
Disassembly of section .init:
0000000080000000 <_start>:
80000000: 00002197 auipc gp,0x2
80000004: 88418193 addi gp,gp,-1916 # 80001884 <__global_pointer$>
80000008: 08000117 auipc sp,0x8000
8000000c: ff810113 addi sp,sp,-8 # 88000000 <__stack_top>
80000010: 00010433 add s0,sp,zero
80000014: 0040006f j 80000018 <main>
Disassembly of section .text:
0000000080000018 <main>:
80000018: 1101 addi sp,sp,-32
8000001a: ec22 sd s0,24(sp)
8000001c: 1000 addi s0,sp,32
8000001e: 4785 li a5,1
80000020: fef42623 sw a5,-20(s0)
80000024: 4789 li a5,2
80000026: fef42423 sw a5,-24(s0)
8000002a: fec42703 lw a4,-20(s0)
8000002e: fe842783 lw a5,-24(s0)
80000032: 9fb9 addw a5,a5,a4
80000034: fef42223 sw a5,-28(s0)
80000038: fe442783 lw a5,-28(s0)
8000003c: 2785 addiw a5,a5,1
8000003e: 2781 sext.w a5,a5
80000040: fe442703 lw a4,-28(s0)
80000044: 9fb9 addw a5,a5,a4
80000046: fef42223 sw a5,-28(s0)
8000004a: 4781 li a5,0
8000004c: 853e mv a0,a5
8000004e: 6462 ld s0,24(sp)
80000050: 6105 addi sp,sp,32
80000052: 8082 ret
Disassembly of section .eh_frame:
0000000080000058 <__DATA_BEGIN__-0x102c>:
80000058: 0014 0x14
8000005a: 0000 unimp
8000005c: 0000 unimp
8000005e: 0000 unimp
80000060: 00527a03 0x527a03
80000064: 7c01 lui s8,0xfffe0
80000066: 0101 addi sp,sp,0
80000068: 07020d1b addiw s10,tp,112
8000006c: 0001 nop
8000006e: 0000 unimp
80000070: 0010 0x10
80000072: 0000 unimp
80000074: 001c 0x1c
80000076: 0000 unimp
80000078: ff88 sd a0,56(a5)
8000007a: ffff 0xffff
8000007c: 0018 0x18
8000007e: 0000 unimp
80000080: 0000 unimp
...
0x80000052 is the end of main, but stops two bytes before the next section .eh_frame. My guess is that it's some sort of alignment problem, but I'm not sure how to read most of this stuff, so any pointers would be welcome!
I've attached my .ld script and .dts output with this post:
riscv64-virt.dts.txt riscv64-virt.ld.txt
I had a chat with pm215 on #qemu@oftc irc, and he identified the problem to be bios that gets automatically loaded at memory start. Also -kernel adds some magic, making the PC start in a different place (0x1000 on mine).
Adding -bios none and using -device loader,file=<elf-file>,cpu-num=0 instead of using -kernel <elf-file> solves it.
this is the culprit:
https://www.qemu.org/docs/master/system/deprecated.html#risc-v-bios-since-5-1