blob: 44312acffeeccf0ad1b73aa8c1bc0c840db5a641 (
plain) (
blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
risc-v: 0.765
mistranslation: 0.718
device: 0.604
graphic: 0.540
architecture: 0.501
semantic: 0.443
assembly: 0.442
ppc: 0.405
debug: 0.343
vnc: 0.323
hypervisor: 0.278
PID: 0.230
socket: 0.226
kernel: 0.223
performance: 0.190
boot: 0.186
network: 0.177
register: 0.171
permissions: 0.166
arm: 0.148
x86: 0.134
files: 0.131
virtual: 0.111
peripherals: 0.101
user-level: 0.091
i386: 0.090
TCG: 0.085
VMM: 0.069
KVM: 0.052
RISC-V: mtval/stval is not correctly set to the instruction itself on illegal instructions
Description of problem:
QEMU 7.0 claims to support `stval`/`mtval` for illegal instructions, but `mtval`/`stval` is actually set to `0`
Steps to reproduce:
1. Assemble and link `mtval-illegal.elf`. The code simply sets up a trap handler and generates an illegal instruction exception
2. Start QEMU with:
```
qemu-system-riscv64 -cpu rv64,h=off -bios mtval-illegal.elf -nographic -icount shift=0 -s -S
```
3. Attach with GDB:
```
gdb mtval-illegal.elf
# Within GDB
target extended-remote :1234
break trap
disp $mtval
# Keep single stepping until breakpoint
stepi
```
4. When control flow reaches `trap`, `mtval` is written with `0` instead of the encoding of `csrw time, x0` (`0xc0101073`)
Additional information:
Writing `0` to `mtval` on a illegal instruction trap is allowed by the specs, but since the [changelog of QEMU 7.0][changelog] says it should be supported, I would consider it a bug.
[changelog]: https://wiki.qemu.org/ChangeLog/7.0#RISC-V
I encountered this when trying to figure out why my program worked with QEMU 6 but breaks with QEMU 7. It's more complicated, but in that case I managed to get `mtval` written with neither `0` nor the actual illegal instruction, but a different illegal instruction. I will try gathering up all the dependencies and write down the steps to reproduce if needed and if I find the time.
|