[Qemu-devel] Strange behaviour with MSR?

[François <aifsair@gmail.com>]

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Hello,

I'm not sure I'm on the right list to post, sorry about that, but I
tried on IRC and got no answer.

I'm working on a low level piece of system, which has to change PSR
values on ARM.

I use qemu-system-arm v 2.0.0, with the command : qemu-system-arm
-nographic -s -S -m 1024 -M vexpress-a9 -kernel ./bootstrap


My issue is the following: Just before the MSR call, I have an LR value.
When executing MSR, the LR value gets nulled.
After a second iteration, MSR does *not* set this value to 0.

I really don't see wether it can come from qemu, if I'm writting this
message, I think it does not come from my code, since the reset caused
by the lr = 0 restarts the system, and thus, re initilizes the context
with the same values and same call graph.

Here is a gdb trace from the issue :


(gdb) target remote 127.0.0.1:1234
Remote debugging using 127.0.0.1:1234
0x60000000 in ?? ()
(gdb) b *0x61005814
Breakpoint 1 at 0x61005814
(gdb) c
Continuing.

Breakpoint 1, 0x61005814 in ?? ()
(gdb) x /i $pc
=> 0x61005814:  msr     CPSR_fsxc, r3
(gdb) info reg
r0             0xe1a010b2       -509603662
r1             0x0      0
r2             0x2822001        42082305
r3             0xe1a010b2       -509603662
r4             0x0      0
r5             0x6100ec04       1627450372
r6             0x0      0
r7             0x0      0
r8             0x0      0
r9             0x0      0
r10            0x0      0
r11            0x60340be4       1614023652
r12            0x0      0
sp             0x60340bc0       0x60340bc0
lr             0x610057cc       1627412428
pc             0x61005814       0x61005814
cpsr           0x80000013       -2147483629
(gdb) si
0x61005818 in ?? ()
(gdb) info reg
r0             0xe1a010b2       -509603662
r1             0x0      0
r2             0x2822001        42082305
r3             0xe1a010b2       -509603662
r4             0x0      0
r5             0x6100ec04       1627450372
r6             0x0      0
r7             0x0      0
r8             0x0      0
r9             0x0      0
r10            0x0      0
r11            0x60340be4       1614023652
r12            0x0      0
sp             0x0      0x0
lr             0x0      0
pc             0x61005818       0x61005818
cpsr           0xe0000092       -536870766


Thanks in advance for any piece of advice :)

--
François


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