[Qemu-devel] target-s390x: assertion failure in op_risbg

[Peter Maydell <peter.maydell@linaro.org>]

This is from https://bugs.launchpad.net/qemu/+bug/1701798, but
that's quite a large thing, so here's the s390 specific part.

On an ubuntu xenial install:

$ apt install g++-5-s390x-linux-gnu
$ cat hello.c
#include <stdio.h>
int main(void) {
    printf("hello world\n");
    return 0;
}
$ s390x-linux-gnu-gcc-5 -O hello.c -o hello.s390x
$ QEMU_LD_PREFIX=/usr/s390x-linux-gnu/ gdb --args
~/linaro/qemu-from-laptop/qemu/build/all-linux-static/s390x-linux-user/qemu-s390x
./hello.s390x
[...]
(gdb) r
[...]
Thread 1 "qemu-s390x" received signal SIGABRT, Aborted.
0x0000000060215018 in raise ()
(gdb) bt
#0  0x0000000060215018 in raise ()
#1  0x000000006021573a in abort ()
#2  0x0000000060079a96 in op_risbg (s=0x7fffffffda10, o=0x7fffffffd950)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/target/s390x/translate.c:3450
#3  0x0000000060082c8b in translate_one (env=0x627f0350, s=0x7fffffffda10)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/target/s390x/translate.c:5824
#4  0x0000000060082f3f in gen_intermediate_code (cs=0x627e80b0,
    tb=0x60794d40 <static_code_gen_buffer+56064>)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/target/s390x/translate.c:5925
#5  0x00000000600369aa in tb_gen_code (cpu=0x627e80b0, pc=274886359240,
    cs_base=0, flags=3, cflags=0)

This is because in op_risbg() we abort() if s->fields->op2 is not
one of 0x55, 0x5d, 0x51. In this case it is 0x59. I don't know enough
s390 to know what this might be, but we shouldn't really abort()
inside QEMU for unimplemented guest insns.

(if you let execution continue, or don't run in a debugger then
linux-user does its usual thing of obfuscating failures inside QEMU
and then runs into a different assert due to trying to take the tb
lock inside cpu_restore_state().)

thanks
-- PMM