Rom memory overlap error

[nolash]

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

[nolash]

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